Vapochromic coordination polymers for use in analyte detection

ABSTRACT

Vapochromic coordination polymers useful for analyte detection are provided. The vapochromism may be observed by visible color changes, changes in luminescence, and/or spectroscopic changes in the infrared (IR) signature. One or more of the above chromatic changes may be relied upon to identify a specific analyte, such as a volatile organic compound or a gas. The chromatic changes may be reversible to allow for successive analysis of different analytes. The polymer has the general formula M W [M′ X (Z) Y ] N  wherein M and M′ are the same or different metals capable of forming a coordinate complex with the Z moiety; Z is selected from the group consisting of halides, pseudohalides, thiolates, alkoxides and amides; W is between 1-6; X and Y are between 1-9; and N is between 1-5. One embodiment provides [Metal(CN) 2 ]-based coordination polymers with vapochromic properties, such as Cu[Au(CN) 2 ] 2  and Zn[Au(CN) 2 ] 2  polymers.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/169,406 filed 8 Jul. 2008 which is a continuation-in-part of U.S.patent application Ser. No. 11/577,299 filed 17 Oct. 2005, now U.S. Pat.No. 8,008,090, which claims the benefit of U.S. provisional patentapplication Ser. No. 60/618,573 filed 15 Oct. 2004. The disclosure ofeach of the previously referenced patent applications and patent ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This application relates to coordination polymers having vapochromicproperties useful for analyte detection.

BACKGROUND OF THE INVENTION

The controlled design and synthesis of metal-organic coordinationpolymers from the self-assembly of simple molecular building blocks isof intense interest due to the promise of generating functionalmaterials.^(1,2) Vapochromic materials, which display optical absorptionor luminescence changes upon exposure to vapors of analytes, such asvolatile organic compounds (VOCs), have been a focus of attention due totheir potential applications as chemical sensors.³⁻⁹ For example, whenexposed to certain organic solvents, the extended Prussian BlueCo²⁺—[Re₆Q₈(CN)₆]⁴⁻ (Q=S, Se) system yields dramatic changes in thevisible spectrum that are attributable to the sensed solvent impactingthe geometry and hydration around the Co^(II) centers.⁹

Several vapochromic compounds based on Au^(I), Pd^(II), and Pt^(II)coordination polymers have also been reported.³⁻⁸ The vapochromism inthese systems is based on changes in both the visible absorption andemission spectra. In the linear {(T1[Au(C₆Cl₅)₂]}_(n) polymer, weakinteractions between the T1 atoms and the adsorbed VOC molecules modifyslightly the color, and more significantly the emission spectra.⁶ On theother hand, changes in the emission spectra of [Pt(CN—R)₄][M(CN)₄](R=iso-C₃H₇ or C₆H₄—C_(n)H_(2n+1); n=6, 10, 12, 14 and M=Pt, Pd) occurwhen metal-metal distances are modified due to the presence of VOCmolecules in lattice voids; small changes in the absorption spectrum canalso be observed.^(7,10) Another example is the trinuclear Au^(I)complex with carbeniate bridging ligands, for which its luminescence isquenched in the solid-state when C₆F₆ vapor is adsorbed due to thedisruption of Au—Au interactions.⁵

Some of these vapochromic materials have recently been incorporated inchemical sensor devices. For example,[Au—(PPh₂C(CSSAuC₆F₅)PPh₂Me)₂][ClO₄] has been used in the development ofan optical fiber volatile organic compound sensor.¹¹ A vapochromic lightemitting diode¹² and a vapochromic photodiode¹³ have also been builtusing tetrakis(p-dodecylphenylisocyano) platinum tetranitroplatinate andbis(cyanide)-bis(p-dodecylphenylisocyanide)platinum(II), respectively.

In these previous discoveries, slight shifts in the ν_(CN) stretch areobserved if hydrogen-bonding between the N(cyano) atoms and the VOCmolecules present in the lattice occurs. Importantly, VOCs cannot bereadily differentiated or identified via IR spectroscopy in this casesince ν_(CN) shifts of only 0-10 cm⁻¹ are usually observed.^(11,14,15)

To overcome the shortcomings of the prior art, the need has arisen forcoordination polymers having improved vapochromic properties forenhancing the sensitivity of analyte detection. The IR signaturesachieved by the present invention are unusually diagnostic for aparticular analyte, both in the number and position of the IR bands. Inthe case of some gases, the adsorption of the analyte to the polymersubstantially enhances the IR response. That is, the response in theν_(CN) or other pertinent region of the spectrum is extremely strongcompared to the direct IR-signature of some gases, which is the currentstate-of-the-art in gas sensors. Moreover, in the present invention thevapochromism of polymers can be readily and reversibly observed both bymultiple means, such as visible colour changes and luminescence changesin addition to IR spectroscopic changes.

SUMMARY OF THE INVENTION

In accordance with the invention, a vapochromic polymer is describedhaving the general formula M_(W)[M′_(X)(Z)_(Y)]_(N) wherein M and M′ arethe same or different metals capable of forming a coordinate complexwith the Z moiety; Z is selected from the group consisting of halides,pseudohalides, thiolates, alkoxides and amides; W is between 1-6; X andY are between 1-9; and N is between 1-5. For example, in one embodimentW and X are 1 and Y and N are 2.

The vapochromic properties of the polymer change when the polymer isexposed to different analytes. The polymer may therefore be used foranalyte detection. The vapochromism may be observed by visible colorchanges, changes in luminescence, and/or spectroscopic changes in theinfrared IR signature. One or more of the above chromatic changes may berelied upon to identify a specific analyte, such as a volatile organiccompound or a gas. The chromatic changes may be reversible to allow forsuccessive analysis of different analytes using the same polymer.

In alternative embodiments of the invention M may be a transition metal,such as Cu and Zn, M′ may be a metal such as Au, Ag, Hg and Cu, and Zmay be a pseuodohalide, such as CN, SCN, SeCN, TeCN, OCN, CNO and NNN.Optionally, an organic ligand may be bound to M. In one particularembodiment a new class of [Metal(CN)₂]-based coordination polymers withvapochromic properties is described, such as Cu[Au(CN)₂]₂ andZn[Au(CN)₂]₂ polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which describe embodiments of the invention but which shouldnot be construed as restricting the spirit or scope of the invention inany way,

FIG. 1 is a diagram of the 1-D crystal structure of a first polymorph ofCu[Au(CN)₂]₂(DMSO)₂. DMSO-methyl groups were removed for clarity.

FIGS. 2( a) and (b) are diagrams of the 3-D crystal structure of thefirst polymorph of Cu[Au(CN)₂]₂(DMSO)₂.

FIGS. 3( a) and (b) are diagrams of the 2-D and 3-D crystal structure ofa second polymorph of Cu[Au(CN)₂]₂(DMSO)₂.

FIG. 4 is a graph showing the thermal stability of the first and secondpolymorphs of Cu[Au(CN)₂]₂(DMSO)₂.

FIG. 5 are photographs showing the vapochromic behavior of the secondpolymorph of Cu[Au(CN)₂]₂(DMSO)₂ after exposure to various analytes,namely DMSO, water, MeCN, DMF, Dioxane, Pyridine and NH₃.

FIGS. 6( a) and (b) are diagrams of the 2-D and 3-D crystal structure ofCu[Au(CN)₂]₂(DMF).

FIG. 7( a) and (b) are diagrams of the 2D crystal structure ofCu[Au(CN)₂]₂(pyridine)₂.

FIG. 8 is a diagram of the postulated 2-D crystal structure of a solventfree complex of Cu[Au(CN)₂]₂.

FIG. 9 are photographs showing changes in luminescence in theZn[Au(CN)₂]₂(analyte)_(x) system (top—under room light; bottom—under UVlight). From left to right: Analyte=None, NH₃, pyridine, CO₂, DMSO.

FIG. 10 is a spectrograph showing the comparative IR spectra in thecyanide region for three analytes (solvents), namely pyridine, DMF andwater using the Cu[Au(CN)₂]₂(solvent)_(x) polymer.

FIG. 11 is a series of photographs showing the structure of Zn[Au(CN)₂]₂polymorph crystals.

FIGS. 12( a) and 12(b) are diagrams of the crystal structure of an αpolymorph of Zn[Au(CN)₂]₂.

FIGS. 13( a), 13(b) and 13(c) are diagrams of the crystal structure ofan β polymorph of Zn[Au(CN)₂]₂.

FIGS. 14( a), 14(b) and 14(c) are diagrams of the crystal structure ofan γ polymorph of Zn[Au(CN)₂]₂.

FIG. 15 is a graph showing a difractogram of the γ polymorph ofZn[Au(CN)₂]₂.

FIGS. 16( a), 16(b), 16(c) and 16(d) are diagrams of the crystalstructure of an δ polymorph of Zn[Au(CN)₂]₂.

FIG. 17 is a series of graphs showing excitation spectra for the α, β,and γ polymorphs of Zn[Au(CN)₂]₂.

FIG. 18 is a series of photographs of powder of the α polymorph ofZn[Au(CN)₂]₂ under UV and visible light before and after exposure toammonia.

FIGS. 19( a) and 19(b) are graphs showing excitation and emissionspectra of (a) {Zn(NH₃)₂[Au(CN)₂]₂} and (b) fully saturated{Zn(NH₃)₄][Au(CN)₂]₂}.

FIG. 20 is a graph showing simulated and observed powder difractogramsof {Zn(NH₃)₂[Au(CN)₂]₂}.

FIGS. 21( a) and 21(b) are diagrams of the crystal structure of{Zn(NH₃)₂[Au(CN)₂]₂}.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the present invention.Accordingly, the specification and drawings are to be regarded in anillustrative, rather than a restrictive, sense.

This application relates to vapochromic polymers useful for detection ofanalytes. The polymers have the general formula M_(W)[M′_(X)(Z)_(Y)]_(N)wherein M and M′ are the same or different metals capable of forming acoordinate complex in conjunction with the Z moiety; Z is selected fromthe group consisting of halides, pseudohalides, thiolates, alkoxides andamides; W is between 1-6; X and Y are between 1-9; and N is between 1-5.As will be apparent to a person skilled in the art and as describedherein, the vapochromic polymers of the invention may also compriseother constituents including ligands, counterbalancing ions and othermetals. The invention encompasses polymers having the same empiricalformula as set out above which exhibit vapochromic properties.

As described below, the vapochromism of the polymers may be observed by(1) visible changes, such as changes in colour or luminescence uponexposure to analytes, or by (2) infrared (IR) spectroscopic changes. Theinvention thus provides two means or “channels” to thereby achievehighly sensitive analyte detection. As used in this patent applicationthe term “vapochromic” refers to a material that has a spectroscopicproperty change upon exposure to a liquid or the vapour of a volatileliquid or gas and the term vapochromism refers to such a spectroscopicproperty change. The spectroscopic property may include any wavelengthof light including microwaves, infrared, visible colour andluminescence. As used in this patent application the process of“detecting chromatic changes” includes detecting a spectroscopicproperty change, including both visible and non-visible changesresulting from exposure to an analyte.

As exemplified by the examples described below, in some embodiments ofthe invention M is a transition metal such as copper (Cu) or zinc (Zn)and M′ is a metal such as gold (Au), silver (Ag), mercury (Hg) orcopper. The Z moiety may be an ion or anionic ligand. Suitable Zmoieties include pseudohalide ions such as CN—. As will be apparent to aperson skilled in the art, other suitable pseudohaldides include SCN,SeCN, TeCN, OCN, CNO and NNN. In particular embodiments of the inventionCu[Au(CN)₂]₂ and Zn[Au(CN)₂]₂ polymers are described. The Cu[Au(CN)₂]₂embodiment takes advantage of the unique chemical properties of gold (I)and copper (II) ions, such as attractive gold-gold interactions andluminescence for gold and a flexible coordination sphere for copper. Theattractive interactions enable the formation of chemically stable,high-dimensionality materials and the gold-luminescence, cyanide-IR andcopper(II) visible spectrum can all act as simultaneous sensory outputs.Similarly, with respect to the Zn[Au(CN)₂]₂ embodiment, distinctiveluminescence and other photochromic qualities are exhibited.

In other embodiments of the invention the metal M may be a 1^(st) rowtransition metal other than Cu or Zn, such as Sc, Ti, V, Cr, Mn, Fe, Co,or Ni, or some other transition-metal such as Zr, Nb or Ru. M may alsobe a lanthanide. Although the Mn (water)¹⁶, Fe (with K-salt)¹⁶, Co(none, with K-salt^(17,18), and DMF¹⁹), Zn (none)²⁰ and a fewlanthanides (Gd, Eu, Yb—all with no ligands)^(21,22-24) complexes areknown in the prior art (ligands shown in brackets), no sensor orvapochromic properties for such complexes have been previouslydescribed.

Optionally, an organic ligand may be bound to M. The ligand may be anyligand capable of capping the metal cation, and may include nitrogen,oxygen, sulfur or phosphorus donors.

Depending upon the resultant charge of the M_(W)[M′_(X)(Z)_(Y)]_(N)structure, a charge-balancing ion, either a cation or anion, may also bepresent. For example, a charge balancing ion may be required where M′ isHg.

In alternative embodiments of the invention the metal M′ may be selectedto produce both linear metal cyanides or non-linear cyanides. Forexample, cyanometallate units such as [Au(CN)₂]⁻, [Ag(CN)₂]⁻ or[Hg(CN)₂] may be incorporated into polymers in conjunction withdifferent transition metal cations and supporting ligands according tothe following general equation:^(25-27,28-33)

[METAL cation+organicligand_(z)]^(n+)+[M′(CN)_(x)]^(y)→[METAL(ligand)_(z)][M′(CN)_(x)]_(n)

-   -   (n=1-5, x=2-9, y=−5 to 0, z=0-9)

The synthesis may be readily accomplished in solvents such as water oralcohols. Compared to prior art approaches, the polymers andpolymer-analyte compositions of the present invention can be preparedfrom extremely simple commercially available starting materials inminimal steps. As described in the Examples section below, the syntheticmethodology, which has built-in design flexibility, low-cost and simplesynthesis, is also a general advantage of coordination polymer systemsover current zeolitic technology. The system is modular in that themetal cation and organic ligand can be chosen as desired to target aparticular application or property.

An important advantage of embodiments of the invention described hereinis that the vapochromic properties of the polymers may be reversible.For example, the Cu[Au(CN)₂]₂ embodiment shows reversible vapochromicsensor behaviour attributable to the Cu—Au pairing.³⁴ Starting with asolid of any Cu[Au(CN)₂]₂(solvent)_(x), addition of a different solventvapour (analytes) generates a new complex. As described below,exceptions may apply in the case of very strong donor solvents such aspyridine or ammonia, which bind strongly to the Cu^(II) center and arenot easily displaced by other solvents. Similarly, the Zn[Au(CN)₂]₂embodiment exhibits reversible vapoluminescent material qualities.³⁵ Thepolymers of the invention may thus be employed in a dynamic system forsuccessively detecting different analytes without the need forreinitialization (although reinitialization may still be required torepeatedly detect the same analyte).

The invention may be used for detecting a wide variety of analytesincluding volatile organic compounds (VOCs) and gases. The solidpolymers adsorb (i.e. bind or trap) analyte, such as organic solvents,exposed to the polymers in a vapour (or liquid) phase. The detectableVOCs typically include a hetero (non-carbon) atom donor such ashydrogen, nitrogen, oxygen, sulfur and phosphorus donors. Examples ofsolvent vapours that will effectively adsorb to the polymers of theinvention include pyridine, dioxane, water, ethanethiol andtrimethylphosphine. Donor gases such as H₂S and ammonia also readilybind and are detectable. The binding capacity and sensitivity of thepolymers may be adjusted through altering the identity of the metals Mand M′ to enable detection of a range of gases, including but notlimited to NO_(x), SO_(x), CO_(x) and alkenes. For example, thezinc-based polymer described herein appears to bind CO₂ and may haveapplications as a CO₂-sensor.

As will be appreciated by a person skilled in the art, the polymers ofthe invention may find application in wide range of industrial andcommercial applications, such as in the chemical, energy andenvironmental sectors. The polymers may be used in many different solidforms depending upon the vapochromic application, such as powders,crystals, thin films or combinations thereof. Exemplary industrialapplications include: personal and badge monitors in chemicallaboratories (e.g. industrial chemical or pharmaceutical researchlaboratories, paint and coatings manufacturing, cosmetics manufacturing)for hazardous vapour detection; portable or stationary thresholdmonitors for chemical vapours in laboratory environments or chemicalstorage facilities for hazardous vapour detection or regulated emissionrequirements; environmental sensor for volatile organic compounds orgases (“electronic noses”) for use at environmental remediation sites,landfills, air-quality monitoring etc.; and responsive coatings, artsupplies, colour-changing paint and other related applications where acolour-changing material is desired.

As will be apparent to a person skilled in the art, the vapochromicpolymers described herein may be deployed in various different forms andapplications for specifically detecting ammonia. For example, thepolymers may be used in medical applications for sensing ammonia in thebreath of patients. In one embodiment, a polymer may be embedded in apaper strip, similar to litmus paper, or onto a binding agent such assilica, which a patient would be instructed to breathe on. Many medicalconditions, such as ulcers, kidney disease and liver disease, areassociated with abnormally high (but still low in an absolute sense)levels of ammonia in the breath of some affected patients. Many otherpotential applications for detecting low levels of ammonia may also beenvisioned.

Although the present invention has been principally described inrelation to analyte sensing and detection, the polymers and compositionsdescribed herein may be useful for other purposes such as extraction,purification and storage applications.

EXAMPLES

The following examples will further illustrate the invention in greaterdetail although it will be appreciated that the invention is not limitedto the specific examples.

The following description of experimental details and experimentalresults is presented in multiple parts. Example 1.0 describes syntheticprocedures and experimental results for the Cu[Au(CN)₂]₂(solvent)_(x)system. Example 2.0 describes a similar synthetic procedure andexperimental results for an analogous Zn[Au(CN)₂]₂(solvent)_(x) system.

Example 1.0 Cu-Based Polymers 1.1 Cu[Au(CN)₂]₂(Solvent)_(x) System 1.1.1Experimental Apparatus and General Procedure

General Procedure and Physical Measurements. All manipulations wereperformed in air. All the reagents were obtained from commercial sourcesand used as received. Infrared spectra were recorded as KBr pressedpellets on a Thermo Nicolet Nexus 670 FT-IR spectrometer. Microanalyses(C, H, N) were performed at Simon Fraser University. Magneticsusceptibilities were measured on polycrystalline samples at 1 T between2 and 300 K using a Quantum Design MPMS-5S SQUID magnetometer. All datawere corrected for temperature independent paramagnetism (TIP), thediamagnetism of the sample holder, and the constituent atoms (by use ofPascal constants).³⁶ Solid-state UV-visible reflectance spectra weremeasured using an Ocean Optics SD2000 spectrophotometer equipped with atungsten halogen lamp. Thermogravimetric analysis (TGA) data werecollected using a Shimadzu TGA-50 instrument in an air atmosphere.

1.1.2 Synthetic Procedures

Synthesis of Cu[Au(CN)₂]₂(DMSO)₂, 1: A 0.5 mL dimethylsulfoxide (DMSO)solution of Cu(ClO₄)₂.6H₂O (0.037 g, 0.1 mmol) was added to a 0.5 mLDMSO solution of KAu(CN)₂ (0.057 g, 0.2 mmol). Green crystals ofCu[Au(CN)₂]₂(DMSO)₂ were obtained by slow evaporation over several days,filtered and air-dried. Yield: 0.050 g, 70%. Anal. Calcd. forC₈H₁₂N₄Au₂CuO₂S₂: C, 13.39; H, 1.69; N, 7.81. Found: C, 13.43; H, 1.72;N, 7.61. IR (KBr): 3005(w), 2915(w), 2184(s), 2151(m), 1630(w), 1426(w),1408(w), 1321(w), 1031(m), 993(s), 967(m), 720(w), 473(m) cm⁻¹. The sameproduct can be obtained by absorption of DMSO by Cu[Au(CN)₂]₂(H₂O)₂.

Synthesis of Cu[Au(CN)₂]₂(DMSO)₂, 2: A 0.2 mL DMSO solution ofCu(ClO₄)₂.6H₂O (0.037 g, 0.1 mmol) was added to a 0.4 mL DMSO solutionof KAu(CN)₂ (0.057 g, 0.2 mmol). Blue needles of Cu[Au(CN)₂]₂(DMSO)₂formed after one hour and were filtered and dried under N₂. Yield: 0.057g, 80%. Anal. Calcd. for C₈H₁₂N₄Au₂CuO₂S₂: C, 13.39; H, 1.69; N, 7.81.Found: C, 13.50; H, 1.76; N, 7.62. IR (KBr): 3010(w), 2918(w), 2206(m),2194(s), 2176(m), 2162(m), 1631(w), 1407(w), 1316(w), 1299(w), 1022(m),991(s), 953(m), 716(w), 458(m) cm⁻¹.

Synthesis of Cu[Au(CN)₂]₂(DMF), 3: A 2 mL N,N-dimethylformamide (DMF)solution of Cu(ClO₄)₂.6H₂O (0.037 g, 0.1 mmol) was prepared. Thissolution was added to a 3 mL DMF solution of KAu(CN)₂ (0.057 g, 0.2mmol). A dark blue-green mixture of powder and crystals ofCu[Au(CN)₂]₂(DMF) was obtained after several days of slow evaporationand was filtered and air-dried. Yield: 0.033 g, 52%. Anal. Calcd forC₇H₇N₅Au₂CuO: C, 13.25; H, 1.11; N, 11.04. Found: C, 13.26; H, 1.11; N,11.30. IR (KBr): 2927(w), 2871(w), 2199(s), 2171(shoulder), 1665(s),1660(s), 1492(w), 1434(w), 1414(w), 1384(m), 1251(w), 1105(w), 674(w),516(w), 408(w) cm⁻¹. Single crystals of 3 were obtained by dissolvingCu[Au(CN)₂]₂(H₂O)₂ (5) in DMF and allowing the solution to evaporatevery slowly. The single crystals and the crystal/powder mixture asprepared above had identical IR spectra. The same product can also beobtained by vapour absorption of DMF by severalCu[Au(CN)₂]₂(solvent)_(x) complexes.

Synthesis of Cu[Au(CN)₂]₂(pyridine)₂, 4: A 10 mL pyridine/water/methanol(5:47.5:47.5) solution of Cu(ClO₄)₂.6H₂O (0.111 g, 0.3 mmol) wasprepared. This solution was added to a 10 mL pyridine/water/methanol(5:47.5:47.5) solution of KAu(CN)₂ (0.171 g, 0.59 mmol). A blue powderof Cu[Au(CN)₂]₂(pyridine)₂ was obtained immediately and was filtered andair-dried. Yield: 0.163 g, 75%. Anal. Calcd for C₁₄H₁₀N₆Au₂Cu: C, 23.36;H, 1.40; N, 11.68. Found: C, 23.52; H, 1.44; N, 11.58. IR (KBr):3116(w), 3080(w), 2179(s), 2167(s), 2152(s), 2144(m), 1607(m), 1449(m),1445(s), 1214(m), 1160(w), 1071(m), 1044(w), 1019(m), 758(s), 690(s),642(m) cm⁻¹. Single crystals of 4 were obtained by slow evaporation ofthe remaining solution. The crystals and powder had identical IRspectra. The same product can also be obtained by vapour absorption ofpyridine by several Cu[Au(CN)₂]₂(solvent)_(x) complexes.

Synthesis of Cu[Au(CN)₂]₂(H₂O)₂, 5: A 10 mL aqueous solution ofCu(ClO₄)₂.6H₂O (0.259 g, 0.7 mmol) was prepared and added to a 10 mLaqueous solution of KAu(CN)₂ (0.403 g, 1.4 mmol). A pale green powder ofCu[Au(CN)₂]₂(H₂O)₂ formed immediately and was filtered and air-dried.Yield: 0.380 g, 91%. The same product can be obtained by vapourabsorption of water by several Cu[Au(CN)₂]₂(solvent)_(x) complexes.Anal. Calcd for C₄H₄N₄Au₂CuO₂: C, 8.04, H, 0.67, N, 9.38. Found: C,8.18; H, 0.71; N, 9.22. IR (KBr): 3246(m), 2217(s), 2194(vw), 2171(s),1633(w) cm⁻¹.

Synthesis of Cu[Au(CN)₂]₂, 6: Cu[Au(CN)₂]₂(H₂O)₂ was heated (150° C.) invacuo to yield green-brown Cu[Au(CN)₂]₂. The yield is quantitative, withno ν_(CN) peaks for hydrated 5 observable. Anal. Calcd for C₄N₄Au₂Cu: C,8.56, H, 0, N, 9.98. Found: C, 8.68, H, trace, N, 9.80. IR (KBr):2191(s), 1613(vw), 530(m) cm⁻¹.

Synthesis of Cu[Au(CN)₂]₂(CH₃CN)₂, 7: A 1 mL CH₃CN solution ofCu(ClO₄)₂.6H₂O (0.037 g, 0.1 mmol) was prepared and added to a 2 mLCH₃CN solution of KAu(CN)₂ (0.057 g, 0.2 mmol). A green powder ofCu[Au(CN)₂]₂(CH₃CN)₂ precipitated immediately along with a white powderof KClO₄. To prevent the replacement of CH₃CN by atmospheric water, thesolvent was removed under vacuum and the KClO₄ side product was notremoved through washing and filtering. Anal. Calcd forCu[Au(CN)₂]₂(CH₃CN)₂+2(KClO₄) (C₈H₆N₆Au₂Cl₂CuK₂O₈): C, 10.44; H, 0.65;N, 9.12. Found: C, 10.99; H, 0.57; N, 8.69. IR (KBr): 2297(w), 2269(w),2192(s), 1600(w), 1445(w), 1369(w), 1088(s), 941(w), 925(w), 752(w),695(w), 626(m), 512(w), 468(w), 419(w) cm⁻¹. The same product (withoutKClO₄) can be obtained by vapour absorption of acetonitrile byCu[Au(CN)₂]₂(DMSO)₂ (1 or 2).

Synthesis of Cu[Au(CN)₂]₂(dioxane)(H₂O), 8: A 2 mL dioxane/water (2:1)solution of Cu(ClO₄)₂.6H₂O (0.037 g, 0.1 mmol) was prepared. Thissolution was added to a 4 mL dioxane/water (2:1) solution of KAu(CN)₂(0.057 g, 0.2 mmol). A pale blue-green powder ofCu[Au(CN)₂]₂(dioxane)(H₂O) was obtained immediately and was filtered andair-dried. Yield: 0.057 g, 85%. The same product can be obtained byvapour absorption of dioxane by several Cu[Au(CN)₂]₂(solvent)_(x)complexes (the water molecule included in this case is from ambientmoisture). Anal. Calcd for C₈H₁₀N₄Au₂CuO₃: C, 14.39; H, 1.51; N, 8.39.Found: C, 14.31; H, 1.21; N, 8.43. IR (KBr): 2976(m), 2917(m), 2890(w),2862(m), 2752(w), 2695(w), 2201(s), 2172(w), 1451(m), 1367(m), 1293(w),1255(s), 1115(s), 1081(s), 1043(m), 949(w), 892(m), 871(s), 705(w),610(m), 515(m), 428(m) cm⁻¹.

Synthesis of Cu[Au(CN)₂]₂(NH₃)₄, 9: This product was obtained by vapourabsorption of NH₃ by several Cu[Au(CN)₂]₂(solvent)_(x) complexes. Theyield is quantitative as shown by IR. Anal. Calcd for C₄H₁₂N₈Au₂Cu: C,7.63, H, 1.92, N, 17.80, found: C, 7.56; H, 1.98; N, 17.71. IR (KBr):3359(s), 3328(s), 3271(s), 3212(m), 3182(m), 2175(m), 2148(s), 1639(m),1606(m), 1243(s), 685(s), 435(w) cm⁻¹.

1.1.3 X-ray Crystallographic Analysis

X-Ray Crystallographic Analysis. Cu[Au(CN)₂]₂(DMSO)₂ 1 and 2,Cu[Au(CN)₂]₂(DMF) 3 and Cu[Au(CN)₂]₂(pyridine)₂ 4: Crystallographic datafor all structures are collected in Table 1. Crystals 1, 3 and 4 weremounted on glass fibers using epoxy adhesive and crystal 2 was sealed ina glass capillary. Crystal 1 was a green rectangular plate(0.09×0.12×0.3 mm³), crystal 2 was a pale blue needle (0.11×0.11×0.2mm³), crystal 3 was a green needle (0.09×0.09×0.15 mm³) and crystal 4was a dark blue platelet (0.02×0.06×0.15 mm³).

For 1, data in the range 4°<2θ<55° were recorded using thediffractometer control program DIFRAC³⁷ and an Enraf Nonius CAD4Fdiffractometer. The NRCVAX Crystal Structure System was used to performpsi-scan absorption correction (transmission range: 0.0301-0.1726) anddata reduction, including Lorentz and polarization corrections.³⁸ Allnon-hydrogen atoms were refined anisotropically. Full matrixleast-squares refinement (1231 reflections included) on F (93parameters) converged to R₁=0.042, wR₂=0.047 (I_(o)>2.5σ(I_(o))).

For 2, 3 and 4, data in the ranges 6.9°<2θ<136.1°, 9.2°<2θ<144.0° and12.0°<2θ<142.6° respectively were recorded on a Rigaku RAXIS RAPIDimaging plate area detector. A numerical absorption correction wasapplied (transmission range: 0.019-0.161, 0.0070-0.0199 and0.3484-0.5826) and the data were corrected for Lorentz and polarizationeffects.³⁹ For 2, the Au, Cu and S atoms were refined anisotropically,while the remainders were refined isotropically. For 3 and 4, allnon-hydrogen atoms were refined anisotropically. Full matrixleast-squares refinement on F was performed on 2, 3 and 4, the dataconverging to the following results: for 2, R₁=0.062, wR₂=0.082(I_(o)>3.0σ(I_(o)), 2026 reflections included, 205 parameters); for 3,R₁=0.0315, wR₂=0.0456 (I_(o)>3.0σ(I_(o)), 1538 reflections included, 148parameters); for 4, R₁=0.0276, wR₂=0.0401 (I_(o)>3.0σ(I_(o)), 1021reflections included, 107 parameters).

All structures were refined using CRYSTALS.⁴° The structures were solvedusing Sir 92 and expanded using Fourier techniques. Hydrogen atoms wereincluded geometrically in all structures but not refined. Diagrams weremade using Ortep-3 (version 1.076)⁴¹ and POV-Ray (version 3.6.0)⁴².Selected bond length and angles for 1-4 are reported in Tables 2 to 5respectively.

1.1.4 Results

Synthesis. The reaction of Cu^(II) salts with KAu(CN)₂ indimethylsulfoxide (DMSO) produced two different compounds, depending onthe total concentration of starting reagents. In dilute solution, greencrystals of polymorph 1 formed slowly, whereas blue crystals ofpolymorph 2 were obtained rapidly in a highly concentrated solution. TheIR spectra of 1 and 2 show different features (Table 6); thehigher-energy bands likely correspond to bridging CN-groups, while thelower-energy bands are due to either free or loosely bound CN-groups.⁴³The X-ray crystal structures of 1 and 2 revealed two different polymericnetworks, both with the same empirical formula Cu[Au(CN)₂]₂(DMSO)₂, asconfirmed by elemental analysis.

Crystal Structure of the Green Cu[Au(CN)₂]₂(DMSO)₂ Polymorph, 1. Thefive-coordinate Cu^(II) center in 1 has a τ-value⁴⁴ of 0.44, where τ=0is pure square pyramidal and τ=1 is pure trigonal bipyramidal,suggesting that the coordination geometry could be considered equallydistorted from either polyhedron. The Cu^(II) center is bound to twoDMSO-O atoms (O—Cu—O=167.06°) and three N(cyano) atoms (FIG. 1).Selected bond lengths and angles for 1 are listed in Table 2. Theasymmetric unit contains two different [Au(CN)₂]⁻ units: aCu^(II)-bridging moiety that generates a 1-D chain, and a Cu^(II)-bounddangling group. The chains stack on top of each other parallel to the(101)-plane, forming stacks of chains that are offset to allowinterdigitation of the dangling [Au(CN)₂]⁻ units. Each chain isconnected to the four neighbouring chains through Au—Au interactions of3.22007(5) Å between the Au(1) atoms of each dangling group and theAu(2) atoms of the chain backbone (FIG. 2 (a)). The DMSO moleculesoccupy the channels between the chains; these channels are delineated byboth [Au(CN)₂] groups and Au—Au bonds (FIG. 2( b)). A viable Au—Auinteraction is considered to exist when the distance between the twoatoms is less than 3.6 Å, the sum of the van der Waals radii for gold.⁴⁵

Crystal Structure of the Blue Cu[Au(CN)₂]₂(DMSO)₂ Polymorph, 2. Thestructure of polymorph 2 contains Cu^(II) centers in a Jahn-Tellerdistorted octahedral geometry, with the two DMSO molecules bound in acis-equatorial fashion (O—Cu—O=95.2°) rather than in the nearly180°-arrangement in 1. Selected bond lengths and angles for 2 are foundin Table 3. The four remaining sites (two axial and two equatorial) areoccupied by N(cyano) atoms of bridging [Au(CN)₂]⁻ units, generatingcorrugated 2-D sheets (FIG. 3( a)). These 2-D layers stack (FIG. 3( b))and are held together by weak Au(1)-Au(2) interactions of 3.419(3) Å andperhaps weak Au(3) . . . Au(4) contacts of 3.592(4) Å. Thus, the colourdifference between the two polymorphs can be attributed to the differentcoordination number and geometry around the Cu^(II) centers. That said,the coarse features of 1, namely the rectangular “channels” filled withDMSO molecules, are also clearly delineated in 2.

Magnetic Properties. As polymorphs 1 and 2 clearly have significantlydifferent solid-state structures, it follows that their physical andchemical properties may also vary; this is obviously the case for theirsolid-state optical reflectance spectra, which show λ_(max) of 550±7 and535±15 nm respectively (Table 6). To explore this key issue, a series ofrepresentative properties were investigated. For example, the magneticsusceptibilities of 1 and 2 were measured at temperatures varying from300 to 2 K. At 300 K, μ_(eff)=1.98 and 1.93μ_(B) for 1 and 2respectively, typical for Cu^(II) centers.⁴⁶ As the temperature drops,μ_(eff) decreases and reaches 1.74 and 1.67μ_(B) at 2 K for 1 and 2respectively. There is no maximum in either χ_(M) vs T plot. Thisbehaviour is consistent with weak antiferromagnetic coupling, probablymediated by the diamagnetic Au^(I) center.^(25-27,47) Thus, the twopolymorphs have similar magnetic properties.

Thermal stability. Examining the thermal stabilities of 1 and 2 bythermogravimetric analysis (FIG. 4), 1 loses its first DMSO moleculefrom 150-190° C. and the other one from 210-250° C. For polymorph 2(which has 4 crystallographically distinct DMSO molecules), the firsttwo DMSO molecules are lost between 100-135° C. and then 150-190° C.,while the two remaining DMSO molecules dissociate around 210-250° C.,comparable with 1. Both polymorphs are then stable until ˜310° C., atwhich point cyanogen (C₂N₂) is released, consistent with thedecomposition of the Cu[Au(CN)₂]₂ framework.⁴⁸ Hence, the thermalstabilities of the two polymorphs with respect to the loss of the firstDMSO molecules are significantly different. Differential scanningcalorimetry shows no evidence for the thermal interconversion in thesolid state from 2 to 1 below the decomposition temperature of 2.

Vapochromic Behavior. Interestingly, even though both polymorphs arethermally stable up to at least 100° C., the DMSO molecules can easilybe replaced by ambient water vapour at room temperature to yieldCu[Au(CN)₂]₂(H₂O)₂ (5), as shown by elemental and thermogravimetricanalysis. Despite the fact that both polymorphs have differentsolid-state structures, IR spectroscopy and powder X-ray diffractionshow that both polymorphs convert to the same Cu[Au(CN)₂]₂(H₂O)₂ (5)complex (Table 6). This conversion is reversible. However, if DMSOvapour is added back to 5, only the green polymorph Cu[Au(CN)₂]₂(DMSO)₂(1) is formed, even if the original DMSO-complex to which H₂O was addedwas the blue polymorph (2). The exchange of DMSO for H₂O can be observedvisually from the associated colour change (FIG. 5).

Cu[Au(CN)₂]₂(DMSO)₂ (either 1 or 2) also displays vapochromic behaviourwhen exposed to a variety of other donor solvent vapours (i.e. analytes)in addition to H₂O. Each Cu[Au(CN)₂]₂(solvent)_(x) complex can bedistinguished easily by its colour (FIG. 5 and Table 6). In addition,the ν_(CN) region of the IR spectrum for each solvent complex is acharacteristic, sensitive signature for that solvent (Table 6). FIG. 10is a spectrograph showing the comparative IR spectra in the cyanideregion for three solvents (i.e. analytes), namely pyridine, DMF andwater using the Cu[Au(CN)₂]₂(solvent)_(x) polymer. FIG. 10 showgraphically the characteristic, sensitive signature for each solvent inthe ν_(CN) region of the IR spectrum. Thus both the visible colourchanges and the cyanide-IR changes are dramatic and distinctive for eachanalyte, allowing for more specific and sensitive analyte detection.

Importantly, this solvent exchange is completely reversible, thuspermitting dynamic solvent sensing. As indicated in the above syntheticexamples, starting with a solid of Cu[Au(CN)₂]₂(solvent)_(x), additionof a different solvent vapour generates a new complex. The onlyexceptions occur in the case of very strong donor solvents such aspyridine or ammonia, which bind strongly to the Cu^(II) center and arenot easily displaced by other solvents.

Each Cu[Au(CN)₂]₂(solvent)_(x) complex was also synthesized by reactingCu(II) salts with [Au(CN)₂]⁻ in the appropriate solvent and each wasfound, by elemental analysis, IR spectroscopy, TGA, and crystallography,to be identical to the complex generated by solvent exchange. In everycase, elemental analysis and TGA (Table 7) indicate that the number ofsolvent molecules incorporated into the complex per transition metalcenter is always the same as the number incorporated by vapouradsorption. This is easily rationalized by the fact that all adsorbedsolvent molecules are ligated to the Cu^(II) center in a 1:1, 1:2 or, inthe case of ammonia, a 1:4 ratio, with no additional loosely trappedsolvent molecules in channels (as shown by TGA, Table 7), as is oftenobserved in other porous systems that include solvent.⁴⁹⁻⁵²

Crystal Structure of Cu[Au(CN)₂]₂(DMF), 3. In order to better understandthe structural changes that occur during a vapochromic response of theDMSO polymorphs, the structures of Cu[Au(CN)₂]₂(DMF) (3) andCu[Au(CN)₂]₂(pyridine)₂ (4) were investigated. The structure of 3contains Cu^(II) centers with a square-pyramidal geometry, where thefour basal sites are occupied by N(cyano) atoms of bridging [Au(CN)₂]⁻units and the apical site is occupied by an O-bound DMF molecule.Selected bond lengths and angles for 3 are listed in Table 4. Thealternation of Cu^(II) centers and [Au(CN)₂]⁻ units generates a 2-Dsquare grid motif with all the DMF molecules pointing either above orbelow the plane of the sheet (FIG. 6( a)). This grid is similar to thatobserved in the blue Cu[Au(CN)₂]₂(DMSO)₂ complex (2) if one DMSOmolecule was removed and the corrugation reduced. The layers stack ontop of each other in an offset fashion, thereby disrupting any channels,and are held together by Au(1)-Au(1*^(a)) and Au(2)-Au(2*^(b))interactions of 3.3050(12) Å and 3.1335(13) Å (FIG. 6( b)).

Crystal Structure of Cu[Au(CN)₂]₂(pyridine)₂, 4. The structure of 4 issimilar to that of 3, except that the Cu^(II) centers are surrounded bytwo solvent molecules, generating octahedrally coordinated metals. Theaxial sites and two of the equatorial sites are occupied by N(cyano)atoms of bridging [Au(CN)₂]⁻ units. Pyridine molecules occupy the twoother equatorial sites. Selected bond lengths and angles for 4 arelisted in Table 5. As observed for 3, infinite 2-D layers are obtained(FIGS. 7( a) and (b)). No aurophilic interactions are present betweenthe Au atoms of neighboring sheets, but π-π interactions of ˜3.4 Å arefound between stacked pyridine rings of adjoining sheets. Thus, thesquare-grid array present in 2 and 3 is maintained but in this case thesheets are completely flat, as opposed to the corrugated array found in2. The 180° disposition of the pyridine rings (vs. the cis orientationof the DMSO molecules in 2) also serves to separate the sheets,disrupting potential intersheet Au—Au interactions.

Solvent free Cu[Au(CN)₂]₂, 6. The green-brown solvent-free complex,Cu[Au(CN)₂]₂ (6), was also prepared by thermally removing in vacuo thewater molecules from 5. Changes in the powder X-ray diffractogram and inthe ν_(CN) peaks of 6 indicate that some rearrangement in the frameworkoccurred. The IR spectrum only shows one stretching frequency (2191cm⁻¹), indicating that all CN groups are in a similar environment,reminiscent of the Cu[Au(CN)₂]₂(DMF) structure. This is also comparablewith the results published for the Mn[Au(CN)₂]₂(H₂O)₂ ¹⁶ and theCo[Au(CN)₂]₂(DMF)₂ ¹⁹ systems (which show stretches at 2150 and 2179cm⁻¹ respectively). In these two coordination polymers, the M[Au(CN)₂]₂unit (M=Mn or Co) forms 2-D square grids, with solvent molecules hangingabove and below the plane of the sheet. Although the three-dimensionaltopology of Cu[Au(CN)₂]₂ is not known, it likely forms a similar 2-Dsquare grid network with all N(cyano) atoms equatorially bound to asquare planar Cu^(II) center (FIG. 8), as would be generated bystructurally erasing the DMF molecule from 3. The Cu[Au(CN)₂]₂ systemwas found to be only slightly porous by N₂-adsorption measurements,suggesting that the 2-D sheets stack in an offset fashion, likely withsignificant aurophilic interactions, thereby blocking channel formation.Despite this, solvents are still taken up by this system to yield thesame Cu[Au(CN)₂]₂(solvent)_(x) complexes.

Concentration-controlled synthesis of structural isomers of coordinationpolymers Results obtained by X-ray crystallography and elementalanalysis indicate that 1 and 2 of this Example are true polymorphs orsupramolecular isomers, as opposed to pseudopolymorphs that differ byincorporation of varying amounts or identities of co-crystallizedsolvent molecules.^(53,54) As mentioned above, many factors contributeto the preferential formation of one polymorph over another and it canoften be a challenge to control the synthesis of a desired isomer.⁵³⁻⁵⁷Varying crystallization conditions, such as solvent type, startingmaterials, temperature and concentration are often important to ensuregeneration of just one polymorph. For example, crystallizingNi[Au(CN)₂]₂(en)₂ (en=1,2-ethylenediamine) from [Ni(en)₃]Cl₂.2H₂O or[Ni(en)₂Cl₂] generates molecular and 1-dimensional polymorphic materialsrespectively.⁴⁷ Also, it has been shown that metastable polymorphs canbe obtained by rapid crystallization from a supersaturated solution,e.g., via a fast drop in temperature.^(56,58) For example,{Cu[N(CN)₂]₂(pyrazine)}_(n) forms green/blue and blue polymorphs whencrystallized from concentrated and dilute solution respectively.⁵⁹

Similarly, in the Cu[Au(CN)₂]₂(DMSO)₂ system described in this Example,if the total concentration of reagents is below 0.2 M, 1 is formed,while 2 is obtained exclusively from >0.5 M solutions. Theconcentration-controlled synthesis of structural isomers of coordinationpolymers is uncommon relative to examples with molecularsystems.^(59,60) This concentration dependence suggests that green 1 isthe thermodynamic product, while blue 2, which rapidly precipitates fromsolution, is likely a kinetic product. The fact that Cu[Au(CN)₂]₂(H₂O)₂converts exclusively to the green polymorph 1 when adsorbing DMSO isfurther evidence that 1 is the most energetically favorable polymorph.Interestingly, the density of thermodynamically preferred 1 is actuallylower than that of 2. This surprising situation has been observed inother polymorphs.⁶¹ Although it is unclear if this result can beattributed to entropic or enthalpic contributions, it is conceivablethat the formation of shorter Au—Au bonds in 1 relative to 2 could be animportant energetic factor.

Metal-ligand superstructures It has been recognized that a system doesnot need to be porous in order to undergo guest uptake.⁶² For example, aflexible metal-ligand superstructure can dynamically adapt in order toaccommodate a variety of potential guests.⁶²⁻⁶⁸ In this light, theJahn-Teller influenced flexible coordination sphere and the greaterlability of Cu^(II) compared with other transition metals are likelyimportant features of the Cu[Au(CN)₂]₂(solvent)_(x) system. The relatedMn[Au(CN)₂]₂(H₂O)₂ and Co[Au(CN)₂]₂(DMF)₂ systems previously reportedform more rigid frameworks.^(16,19) For these two systems, thermaltreatment is required to remove the guest molecules and yield compoundsexhibiting zeolitic properties. The lability of Cu^(II) in the system ofthe present invention facilitates the reversible exchange of adsorbedsolvent molecules without any thermal treatment required. It also likelyincreases the flexibility of the framework by allowing the breaking andthe reformation of Cu—N(cyano) bonds, thereby adapting to the solventguest present. Gold-gold interactions are probably present in all theCu[Au(CN)₂]₂(solvent)_(x) complexes and help to stabilize the 3D-networkas solvent exchange takes place.

Taking into account the varied structures of theCu[Au(CN)₂]₂(solvent)_(x) complexes, several modes of flexibility withinthe fundamental structural framework, i.e. the two-dimensionalsquare-grid network of the Cu[Au(CN)₂]₂ moiety (FIG. 8), can beidentified. Firstly, the 2-D square-grid can lie entirely flat, as inthe bis-pyridine or mono-DMF complexes 3 and 4, or it can buckle togenerate a corrugated 2-D array, as observed in the blue bis-DMSOpolymorph 2. The extent of this corrugation can even force the partialfragmentation of the square array via the breaking of one Cu—N(cyano)bond, as observed in the green bis-DMSO polymorph 1. Such fragmentationis probably also present in the Cu[Au(CN)₂]₂(NH₃)₄ complex (9); theCu^(II) center in 9 is likely still octahedral, with two Cu—N(cyano)bonds (out of four in the fundamental square-grid structure) breakingcompletely to make way for two additional NH₃ ligands, therebydisrupting the 2-D array. Another mode of flexibility lies in theability of the Cu^(II) center to readily alternate between being five-and six-coordinate, as well as accessing a range of five-coordinategeometries. This adaptability is independent of the extent ofcorrugation: five-coordinate Cu^(II) centers are found in both flat 3and corrugated 1 while six-coordinate centers are present in both flat 4and corrugated 2. Finally, the Jahn-Teller distortions endemic toCu^(II) complexes yield a third mode of flexibility: the arrangement ofequatorial/axial or basal/apical N(cyano) ligands and donor solvents.Again, this pliability is independent of the extent of corrugation: boththe five-coordinate DMF complex 3 and six-coordinate bis-pyridinecomplex 4 contain flat Cu[Au(CN)₂]₂ square-grids, but in 3 the N(cyano)ligands are all basal (and therefore roughly identical in length) whilein 4 two N(cyano) ligands are equatorial and two are axial, leading tosignificantly different Cu—N(cyano) bond lengths. This form ofstructural flexibility is particularly important since substantiallydifferent IR signatures in the cyanide region are generated depending onthe N(cyano) bonding arrangement in the system. Of course, all threemodes of flexibility work in concert to generate the adaptable, dynamicnetwork solid that is ultimately able to bind and sense different donorsolvents.

The source of the vapochromism in the Cu[Au(CN)₂]₂(solvent)_(x) systemdiffers from that of other Au^(I)-containing systems.³⁻⁶Cu[Au(CN)₂]₂(solvent)_(x) shows vapochromism in the visible since eachdonor solvent molecule that is adsorbed binds to the Cu^(II) center andmodifies differently the crystal field splitting. As a consequence, thecolour of the vapochromic compound changes as the d-d absorption bandsshift with donor. In addition to donor identity, the resultingcoordination number (five or six) and specific geometry of the coppercenter also influences the colour of the complexes by altering thesplitting of the d-orbitals.

The [Au(CN)₂]⁻ unit is also a key component of this system since ittelegraphs the changes in solvent bound to the Cu^(II) centers via theν_(CN) stretch. Each Cu[Au(CN)₂]₂(solvent)_(x) has a different IRsignature since every VOC modifies in a different manner the electrondensity distribution around the Cu^(II) center. This influences theamount of π-back bonding from the Cu^(II) center to the CN group, whichin turn is observed in the IR spectrum due to the change in vibrationfrequency.⁴³ Also, the number of bands observed is related to thesymmetry and coordination number of the Cu^(II) centers, as described indetail above.

In summary, it has been illustrated in this Example that, despite theirdifferent solid-state structures, the two Cu[Au(CN)₂]₂(DMSO)₂ polymorphsexhibit the same vapochromic behaviour with respect to sorption ofanalytes such as VOCs. The use of [Au(CN)₂]⁻ as a building block isimportant to the function of this vapochromic coordination polymer.First, it provides the very sensitive CN reporter group that can allowIR-identification of the solvent adsorbed in the materials. Also, Au—Auinteractions via the [Au(CN)₂]⁻ units increase the structuraldimensionality of the system in most cases and probably help providestabilization points for the flexible Cu[Au(CN)₂]₂ framework.

1.2 Cu[Au(CN)₂]—Based Sensing of Ammonia and Amines

In this example coordination polymer Cu[Au(CN)₂]₂(μ-H₂O)₂ (5) was shownto be effective in reversibly sensing ammonia and amines at lowconcentrations.

As indicated above, Cu[Au(CN)₂]₂(μ-H₂O)₂ 5 is a green powder which, uponexposure to vapours of volatile organic compounds, exhibits visiblevapochromism and changes in the ν_(CN) region of the IR spectrum. Inthis example, a finely ground powder of Cu[Au(CN)₂]₂(μ-H₂O)₂ (5)deposited onto a CaF₂ plate was exposed to a series of solvent vapoursin a range of concentrations in order to determine sensitivity levels.Use of the ν_(CN) IR signature was more sensitive than use of the colourchange monitored by UV-Vis solid state reflectance for all analytes;sensitivity limits for a range of VOCs via both detection channels arereported in Table 8.

Consistent with the vapochromic mechanism, which requires that theanalyte ligate to the copper(II) center, 5 was found to be insensitiveto non-coordinating or very weakly coordinating classes of VOCs (withrespect to copper(II)) and related analytes. A representative set ofcompounds (in brackets) covering a range of functional groups such asesters (ethyl acetate), ethers (tetrahydrofuran), ketones (acetone),thiols (tetrahydrothiophene), aldehydes (formaldehyde), saturatedhydrocarbons (pentane), aromatic hydrocarbons (benzene), chloroalkanes(dichloromethane) and even, remarkably, concentrated acetic acid, allgenerated no response from 5, which remained unscathed after exposure tosaturated atmospheres of the respective VOCs. Compound 5 generatedvapochromic responses to weak donors such as DMSO, DMF and MeCN (Table8), with IR-sensitivities in the range of 400-1000 ppm, while strongernitrogen-based donor VOCs and ammonia had much higher sensitivities,from several-hundred ppm down to ppb levels.

Among a series of representative amines, 5 was found to be moresensitive to primary (1°) amines versus secondary and tertiary (2° and3°) amines. This observation is likely attributable to the higher sterichindrance of 2° and 3° amines (or long-chain 1° amines such asbutylamine) restricting their donor ability, rather than theirincreasing vapour pressures; the boiling point of pyridine (115° C.) andethylenediamine (118° C.) are similar, yet their sensitivities are 400ppm vs. 385 ppb respectively. Similarly, although the boiling point ofethylenediamine (118° C.) is significantly higher than diethylamine (55°C.), 5 was much more sensitive towards ethylenediamine. In the case ofpropylamine and butylamine, since their basicity and boiling points arefairly similar, the large decrease in sensitivity (140 ppb vs. 100 ppmrespectively) probably reflects the limited ability of the flexiblecoordination polymer structure to readily adapt to accommodate longerchain alkyl groups.

Indeed, as shown in Table 8 below, 5 was found to be extremely sensitiveto ammonia and low molecular-weight 1°-amines. For example, thedetection limit with the naked eye (by watching the colour change) when5 is exposed to ammonia is as low as 40 ppm; with visible reflectancespectroscopy and using the spectrum of 1 as a background, it drops to230 ppb. By using the ν_(CN) IR spectroscopy channel, ammoniaconcentrations as low as 36 ppb could be detected. Similarly,propylamine could be detected down to 140 ppb.

Titration studies. Titration of 5 with consecutive equivalents ofammonia or propylamine showed that a series of intermediates wereobtained en route to the final products observed upon exposure to alarge excess. Monitored by IR spectroscopy, 5 interacts with ammonia andamines in a similar manner. Titration with two or less equivalents ofNH₃ per Cu(II) centre of 5 causes a decrease in the ν_(CN) bandsattributable to 5 at 2216 and 2171 cm⁻¹ and the formation of two newbands at 2181 and 2151 cm⁻¹. Upon continuing the titration from two tofour equivalents of ammonia, new ν_(CN) peaks at 2175 and 2148 cm⁻¹replaced the 2181/2151 cm⁻¹ set. Finally, further titration (up to 35equiv.) generated the final NH₃-saturated film that exhibited a newν_(CN) band at 2141 with a small band at 2136 cm⁻¹. This titration datasuggests that two intermediates of stoichiometry Cu[Au(CN)₂]₂(NH₃)₂ (9)and Cu[Au(CN)₂]₂(NH₃)₄ (10) are formed on the way towards the finalsaturated product. Note that the IR spectrum of 9 matches that observedfor the low-concentration/high sensitivity tests of 5 with NH₃ (Table8). The presence of two ν_(CN) bands for all intermediates and theproduct also indicates that two different cyanide environments exist inthe solids.

A similar trend was observed for the propylamine titration of 5: oneequivalent of propylamine vapour generated new peaks at 2185 and 2155cm⁻¹, along with the peaks of 5 at 2216 and 2171 cm⁻¹. Addition of asecond equivalent of propylamine greatly increased the peaks at 2185 and2155 cm⁻¹ with a concomitant loss of the peaks for 5. A third equivalentof propylamine generated three new peaks at 2182, 2142 and 2133 cm⁻¹with a decrease in the intensity of the 2185/2155 cm⁻¹ set; these peaksbecame dominant upon addition of a fourth equivalent. Excess propylaminegenerates a peak at 2143 cm⁻¹. The propylamine titration results alsosuggest the presence of two intermediates, namelyCu[Au(CN)₂]₂(CH₃CH₂CH₂NH₂)₂ (11) and Cu[Au(CN)₂]₂(CH₃CH₂CH₂NH₂)₃ (12)prior to saturation.

These titrations were also monitored by visible reflectance spectroscopydue to the drastic colour changes from green to blue-green to deep bluethat occur upon exposure of 5 to ammonia or propylamine. When 5 isexposed to one to two equivalents of ammonia the λ_(max) shifts from 535to 510 nm with one isosbestic point at 522 nm suggesting that there aretwo compounds in equilibrium, most probably the starting material andthe Cu[Au(CN)₂]₂[NH₃]₂ complex. When exposed to three to six equivalentsof ammonia, the λ_(max) shifts up to 470-485 nm with another crudeisosbestic point at approximately 500 nm suggesting that another set oftwo compounds are in equilibrium, most probably the bis- and thetetrakis-ammine coordinated complexes 9 and 10. It was observed that themaxima obtained after exposing 5 to 1 and 2 equivalents of ammonia andallowing it to equilibrate are very close and the same trend is observedwhen exposing it to 3 to 4 equivalents of ammonia, suggesting that thereare two intermediates formed Cu[Au(CN)₂]₂[NH₃]_(x) (x=2, 4) before itsees an excess of ammonia, which generates a maximum at 435 nm.

When 5 was titrated with propylamine vapour, the same blue-shiftingtrend was observed as for the ammonia analyte. Thus, the addition of oneand two equivalents of propylamine shifted the visible reflectancespectrum from 535 to 502 nm, with a clear isosbestic point at 513 nm.Titration with a third equivalent of propylamine caused a further shiftto 462 nm, with a new isosbestic point at 491 nm; a fourth equivalentinitiated no further shift in this case. The presence of two separateisosbestic points and the titration equivalents needed to maximize thebands (two and three equiv.) are consistent with the proposed formationof intermediates 11 and 12. Addition of an excess of propylamine (14equivalents) shifted λ_(max) to 428 nm, generating a dark blue compound.When saturated with ammonia or propylamine, the observed UV-Vis maximaare unique to each analyte, since the ammonia/propylamine binds to thecopper(II) centre and each has a different ligand field strength; thesame can be said for the other strongly-bonding analytes in Table 8.

Kinetics of vapochromic response. Kinetic studies of analyteadsorption/desorption were also performed on the coordination polymer 5by exposing it to an excess of ammonia and propylamine vapour over aperiod of 2 minutes. When exposed to ammonia, one isosbestic point wasobserved at 2175, suggesting that two compounds were in equilibrium. Theν_(CN) stretches were recorded with respect to time after the exposureof compound 5 to ammonia and it was observed that the band of the drycoating at 2171 and 2216 cm⁻¹ shifts to 2181 and 2151 cm⁻¹ first, whichmatches the value obtained for Cu[Au(CN)₂]₂[NH₃]₂ using both IRtitration studies and synthetic studies. With time peaks at 2175 and2148 cm⁻ are formed, which matches the value obtained forCu[Au(CN)₂]₂[NH₃]₄ as described above. After some time, compound 5 seesan excess ammonia and hence a peak at 2141 cm⁻¹ was observed due to thepresence of free Au(CN)₂ ⁻.

A stepwise blue-shifting of the visible absorption band is observed overthe same time-period for ammonia adsorption. However, the presence ofdynamic mixtures of adducts renders the shifting conglomerate bandsdifficult to assign based on their maxima; however, the end-point is thefully saturated system at 435 nm.

Desorption of NH₃ analyte from the copper center is also observed whenthe cell is opened to air at room temperature. Kinetic studies were alsoperformed on the desorption of ammonia over two minutes from a mixtureof 10 and the fully saturated species at 2175/2148 and 2141/2136 cm⁻¹respectively; the bis-ammonia adduct 9 rapidly grew in at 2181/2150cm⁻¹, confirming the loss of two ammonia molecules. A visible colourchange is also observed from blue to green with the final visiblemaximum at 525 nm (a mixture of 5 and 9) over 2 minutes. Note that thedesorption of ammonia is very quick and hence as soon as the cell isopened loss of ammonia commences.

Analogous kinetic studies with propylamine as analyte also showed theformation of intermediates. When compound 5 was exposed to a smallexcess of propylamine, a shift in the IR ν_(CN) stretches of thestarting material with respect to time was observed. The ν_(CN)stretches shifts from 2171 and 2216 cm⁻¹ to 2155 and 2187 cm⁻¹,consistent with the formation of Cu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]₂ (11). Withtime the peak at 2155 cm⁻¹ decreases and the peaks at 2182, 2140 and2133 cm⁻¹ increase, analogous to Cu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]₃ (12).

Desorption of propylamine from the copper centre was also observed whenthe cell containing compound 5 and propylamine vapour was opened to airat room temperature. The IR ν_(CN) stretches forCu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]₃ 12 moved from 2182, 2140 and 2133 cm⁻¹ to2155 and 2187 cm⁻¹, analogous to Cu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]₂ 11, asanalysed by IR titration and synthetic studies. A peak at 2171 cm⁻¹attributable to the hydrate 5 also grew in over time; exposing thesample to a high humidity (68%) atmosphere for two hours, sharp peaks at2171 and 2216 cm⁻¹ were observed with residual peaks at 2155 and 2187cm⁻¹ corresponding to Cu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]₂ 11. However, at lowhumidity, the Cu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]₂ 11 compound is not reversibleunless a burst of heat is applied (to give the anhydrous complex,Cu[Au(CN)₂]₂).

Upon absorption of propylamine by 5, blue shifts occur but, as with theNH₃ case, the presence of a dynamic mixture ofCu[Au(CN)₂]₂[CH₃CH₂CH₂NH₂]_(x) compounds, each with a differentabsorption band, precluded a detailed analysis of the changes; the finalvisible maximum (broad, 425-440 nm) after saturation corresponds to amixture of 11 and the saturated species 12. Upon desorption, the bandred-shifts and broadens but, as is evident from the IR-data, does notrapidly settle to a single compound—it represents a mixture ofbis-propylamine 11 and hydrate 5.

In general, the response time of 5 depends greatly on the analyte (Table8) and is longer with vapour-saturated dimethylformamide,dimethylsulfoxide, pyridine and acetonitrile (minutes to hours) vs.ammonia (<5 seconds). At lower vapour concentrations of analyte, theresponse times increase, to within a minute for NH₃, for example.

Resetting the vapochromic material. Upon standing in an NH₃-freeenvironment at high humidity, 5 loses all bound ammonia and propylaminewithin 2 hours as determined by IR and UV-Vis spectroscopy. However, inorder to quickly and fully liberate bound analyte and thereby fullyreset the sensor, 5 was heated at 160° C. for 150 seconds. Rather thanregenerate 5 exactly, the related anhydrous Cu[Au(CN)₂]₂ complex 6 wasproduced, as confirmed by the single ν_(CN) stretch at 2191 cm⁻¹. Thismaterial reacts in the same fashion with NH₃ as 5 to reform 9 but thesensitivity is three times lower than the original hydrated 5 and itsresponse time is also slower. Thus, in the absence of high humidity or aheating reset, 5 acts primarily as a sensitive one-time dosimeter forammonia and 1°-amines. Using a heating reset, the sensor was repeatedlycycled a further three times with no apparent degradation or change inperformance.

On the other hand, when exposed to more weakly-bound analytes such asDMF, DMSO, pyridine and acetonitrile, the material will reset to theanalyte-free hydrated 5 at room temperature in ambient atmosphere uponremoval of the analyte vapour; the speed of this regeneration depends onthe specific analyte and ambient humidity but range from seconds foracetonitrile to minutes for pyridine. Thus, for these analytes, 5 actsas a reversible sensor; repeating the cycle several times caused noapparent degradation of the material.

The potential cross-interference of different VOCs with ammonia was alsoanalyzed by introducing mixtures of NH₃ and another VOC to 5: only smallalkyl chain 1°-amines interfere with the NH₃ sensory response and eventhen only if they are present at a higher concentration than theammonia. Other coordinating VOCs such as pyridine, acetonitrile, DMF andDMSO do not interfere. A high level of humidity has an impact on thevapochromic response of 5 to weaker donating solvents (DMSO, DMF,pyridine, acetonitrile etc.) but not with NH₃ or small 1° amines.

As will be apparent to a person skilled in the art, the quantitativevalues referred to in this example are guidelines only and are subjectto a margin of error. The detection sensitivity of a particularvapochromic polymer to a particular analyte will vary depending uponvarious factors including atmospheric conditions, the manner in whichthe polymer is deployed and the concentration of the polymer.

Example 2.0 Zn-Based Polymers 2.1 Zn[Au(CN)₂]₂(Solvent)_(x) System

Synthesis of Zn[Au(CN)₂]₂(DMSO)₂. To a 1 mL DMSO solution ofZn(ClO₄)₂(H₂O)₆ (0.032 g, 0.086 mmol) was added KAu(CN)₂ (0.050 g,0.173). Slow evaporation yielded crystals of Zn[Au(CN)₂]₂(DMSO)₂. Anal.Calcd. for C₈H₁₂N₄Au₂O₂S₂Zn: C, 13.35; H, 1.68; N, 7.79%. Found: C,13.50; H, 1.72; N, 8.04%. IR (KBr, cm⁻¹) 3009 (m), 2919(m), 2849(w),2186(s), 2175(s), 1409(m), 1314(m), 1299(m), 1031(m), 1013(s), 1005(s),957(m), 710(w).

Although the structure of a solvent-free Zn[Au(CN)₂]₂ polymer (13) isknown, it is believed that no luminescence or analyte binding propertieshave previously been reported. FIG. 9 consists of photographs showingchanges in luminescence in a Zn[Au(CN)₂]₂(analyte)_(x) system under roomlight (top) and ultraviolet light (bottom). From left to right in thedish, the analyte is None, NH₃, pyridine, CO₂ and DMSO. As in Example1.0 above, the cyanide-IR changes are also dramatic and distinctive foreach analyte.

The zinc-based polymer described herein appears to bind CO₂: Anal.Calcd. for C₅N₄Au₂O₂Zn: C, 9.89; H, 0.00; N, 9.22%. Found: C, 9.73; H,0.00; N, 9.32%. IR (KBr, cm⁻¹): 2192 (s).

2.2 Synthesis, Structure and Photoluminescent Properties of FourPolymorphs of Zn[Au(CN)₂]₂(13) 2.2.1 General Procedures and PhysicalMeasurements

All manipulations were performed in air. [^(n)Bu₄N][Au(CN)₂].½ H₂O wassynthesized as previously described.⁶⁹ All other reagents were obtainedfrom commercial sources and used as received. Infrared spectra wererecorded as pressed KBr pellets on a Thermo Nicolet Nexus 670 FT-IRspectrometer. Microanalyses (C, H, N) were performed at Simon FraserUniversity on a Carlo Erba EA 1110 CHN Elemental Analyzer.Thermogravimetric analysis (TGA) data were collected using a ShimadzuTGA-50 instrument heating at 1° C./min in an air atmosphere.Differential Scanning calorimetry (DSC) measurements were collected on aPerkin Elmer DSC 7 instrument with a Perkin Elmer TAC 7/DX controllerheating at 2° C./min from 25-300° C.

Solid-state luminescence data were collected at room temperature on aPhoton Technology International (PTI) fluorometer, using a Xe arc lampand a photomultiplier detector. Finely ground powder samples were dropcast, from the synthesis solvent, on a quartz plate and placed at anapproximately 45° angle in a quartz cuvette. Ammonia exposureexperiments were conducted under the same conditions, sealing the top ofthe cuvette with parafilm to prevent rapid loss of ammonia gas. Theheadspace of a bottle of concentrated ammonia solution (29.4%) was usedas the source of ammonia gas.

Emission lifetimes data were determined using a customized apparatus(Prof. K. Sakai, Kyushu University) equipped with an Iwatsu DS-4262Digital Oscilloscope and a Hamamatsu R928/C3830 photomultiplier tubecoupled to a Horiba H-20-VIS grating monochromator. The excitationsource was an N2 laser (337 nm) (Usho KEN-1520).

Nitrogen porosity measurements at 77 K of 13α-13δ were carried out on acustom-built vacuum line (Prof. I. Gay, Simon Fraser University) using agas-volumetric measurement technique. Samples of α-δ were pretreated byheating to 150° C. under vacuum in order to degas the sample and removeany surface-bound solvent. A maximum pressure of 80 Torr was used.

The vapochromic behaviour of the polymorphs 13α-13δ was quantified in acustomized chamber of known volume. The visible emission spectrum wasmonitored using an Ocean Optics QE65000 spectrometer with an OceanOptics deuterium/tungsten-halogen DH-2000-FHS source. Titration andsensitivity limit studies were both performed on the differentpolymorphs, by introducing a known amount of NH₃ (as 2.0 M NH₃ solutionin 2-propanol) into the chamber through a septum using an air-tightsyringe.

The sensitivity of the different polymorphs to NH₃ was determined bysequentially introducing known concentrations of NH₃ into the chamberand observing any change in visible emission at 520 nm. The 520 nmwavelength was chosen to ensure that no peak overlap was observed. Theemission spectrum of the appropriate NH₃-free polymorph startingmaterial was used as the background; this yielded higher sensitivitiesthan use of a MgO white background.

The titration studies were performed by adding sequential equivalents ofNH₃ per Zn(II) centre, allowing the material in the chamber toequilibrate for 30 seconds, and then measuring the IR spectrum.

2.2.2 Synthetic Procedures

Although the inventors have experienced no difficulties, perchloratesalts are potentially explosive and should only be used in smallquantities and handled with care.

α-Zn[Au(CN)₂]₂ (13α). A 15 mL aqueous solution containing Zn(ClO₄)₂′6H₂O(37 mg, 0.10 mmol) was added to a 15 mL aqueous solution of KAu(CN)₂ (57mg, 0.20 mmol). Crystals begin to form after two days of slow, partial,evaporation yielding colourless X-ray quality crystals ofα-Zn[Au(CN)₂]₂, (α). Yield: 40 mg (72%). Anal. Calcd for C₄N₄Au₂Zn: C,8.53%; H, 0.00%; N, 9.94%. Found: C, 8.84%; H, 0.00%; N, 10.15%. IR(KBr, cm⁻¹): 2216 (w), 2198 (s), 2158 (w), 517 (m). A similarpreparation (using a different stoichiometry) and the crystal structurehave been previously reported.²⁰

β-Zn[Au(CN)₂]₂ (13β). To a 3 mL acetonitrile solution containingZn(NO₃)₂.6H₂O (30 mg, 0.10 mmol), a 3 mL acetonitrile solution of[^(n)Bu₄N][Au(CN)₂].½ H₂O (100 mg, 0.20 mmol) was added while stirring,yielding an immediate precipitate. The mixture was centrifuged, afterwhich the solvent was removed and the powder allowed to dry overnight.The sample was washed with three portions of acetonitrile (6 mL) throughfilter paper to remove any KNO₃ or unreacted starting material. Theresulting powder was air dried, yielding a white powder ofβ-Zn[Au(CN)₂]₂, (13β). Yield: 40 mg (72%). Anal. Calcd for C₄N₄Au₂Zn: C,8.53%; H, 0.00%; N, 9.94%. Found: C, 8.51%; H, 0.00%; N, 9.78%. IR (KBr,cm⁻¹): 2221 (sh), 2199 (s), 2158 (w), 521 (m).

X-ray quality crystals of 13β can be grown by slow, partial, evaporationof a 20 mL methanol solution of KAu(CN)₂ (10 mL, 57 mg, 0.20 mmol) andZn(ClO₄)₂′6H₂O (10 mL, 37 mg, 0.10 mmol). The crystals and powder hadidentical IR spectra and powder diffractograms. Crystals of KClO₄ areinterspersed with crystals of 13β.

γ-Zn[Au(CN)₂]₂ (13γ). Method 1: To a 1 mL acetonitrile solutioncontaining Zn(ClO₄)₂.6H₂O (37 mg, 0.10 mmol), a 1 mL acetonitrilesolution of [_(n)Bu₄N][Au(CN)₂].½ H₂O (150 mg, 0.30 mmol) was addeddropwise while stirring, yielding an immediate precipitate. The mixturewas centrifuged, after which the solvent was removed and the powderallowed to dry overnight. The sample was washed with three portions ofacetonitrile (2 mL) through filter paper to remove any KClO₄ andunreacted starting material. The resulting powder was air dried,yielding a white powder of γ-Zn[Au(CN)₂]₂, (13γ). Yield: 40 mg (72%).Anal. Calcd for C₄N₄Au₂Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found: C,8.62%; H, 0.00%; N, 9.82%. IR (KBr, cm⁻¹): 2187 (s), 2149 (w), 526 (m).

Method 2: To a 5 mL (99:1) acetonitrile:water solution containingZn(ClO₄)₂.6H₂O (37 mg, 0.10 mmol), a 5 mL (99:1) acetonitrile:watersolution of KAu(CN)₂ (57 mg, 0.20 mmol) was added dropwise whilestirring, yielding an immediate precipitate, which was filtered andwashed with three portions of acetonitrile (3 mL), leaving a powder of13γ. Yield: 43 mg (77%). The IR spectra and X-ray powder diffractogramsof the products from both methods are identical.

δ-Zn[Au(CN)₂]₂ (13δ). A 15 mL aqueous solution containing KAu(CN)₂ (114mg, 0.40 mmol) and KCN (26 mg, 0.40 mmol) was covered with a watch glassand brought to a boil, after which HCl (0.80 mL, 0.100 N solution, 0.080mmol) was added. The solution was cooled until the beaker was warm tothe touch, after which a 15 mL aqueous solution of ZnCl₂ (26 mg, 0.20mmol) was added. Over several hours, crystals of δ-Zn[Au(CN)₂]₂ (13δ)deposited. The crystals were immediately filtered and dried. Yield 60 mg(56%). Anal. Calcd for C₄N₄Au₂Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found:C, 8.44%; H, 0.00%; N, 10.27%. IR (KBr, cm⁻¹): 2191 (s), 2188 (s), 2156(w), 2151 (w), 526 (m). Further evaporation yields a mixture of 13α and13δ crystals, after which a yellow powder of AuCN is produced.

Zn(NH₃)₂[Au(CN)₂]₂} (14). Ammonia gas (10 mL), from the saturatedheadspace of a bottle of concentrated ammonia solution (29.4%), wasintroduced to a vial containing 20 mg of 13β. The vial was sealed andleft standing for 30 min, yielding a white powder. The vial was thenopened and the ammonia allowed to escape for 5 min, generating a bright,yellow powder of {Zn(NH₃)₂[Au(CN)₂]₂} 14. Anal. Calcd. for C₄H₆N₆Au₂Zn:C, 8.04%; H, 1.01%; N, 14.07%. Found: C, 8.30%; H, 0.91%; N, 13.87%. IR(KBr, cm⁻¹): 3290 (m), 3178 (w), 2158 (s), 2117 (sh), 1202 (s), 618(br,m).

Using 13α, 13γ, or 13δ as the starting material still generated{Zn(NH₃)₂[Au(CN)₂]₂} 14.

If a sample of {Zn(NH₃)₂[Au(CN)₂]₂} 14 is left unsealed for an hour, awhite powder of Zn[Au(CN)₂]₂ 13 forms. Anal. calcd. for C₄N₄Au₂Zn: C,8.53%; H, 0.00%; N, 9.94%. Found: C, 8.86%; H, 0.00%; N, 9.80%. IR (KBr,cm⁻¹): 2197 (s), 2160 (w), 518 (m).

2.3.3 X-ray Crystallographic Analysis

Crystallographic data for the four polymorphic compounds 13α-13δ and{Zn(NH₃)₂[Au(CN)₂]₂} 14 are tabulated in Table 9. Crystals of 13β and13δ were mounted on glass fibers using epoxy adhesive. 13β was acolourless plate having dimensions 0.20×0.20×0.02 mm³ and 13δ was acolourless plate having dimensions 0.14×0.11×0.06 mm³. The data forcompounds 13β and 13δ were collected at room temperature on a BrukerSmart instrument with an APEX II CCD area detector at a distance of 6.0cm from the crystal. A Mo Kα fine-focus sealed tube operated at 1.5 kWpower (50 kV, 30 mA) was utilized for data collection. The followingdata ranges were recorded: 13β=4°<2θ<57°; 13δ=7°<2θ<65°.

For compound 13β, a total of 776 frames were collected with a scan widthof 0.5° in ω; all frames were collected with a 20 s exposure time. Theframes were integrated with the Bruker SAINT software package. Data werecorrected for absorption effects using a numerical face-indexedtechnique (SADABS) with a transmission range of 0.058-0.700. Finalunit-cell dimensions were determined based on the refinement of theXYZ-centroids of 1969 reflections with ranges 4°<2θ<51°

Crystals of compound 13δ were determined to be two componentnon-merohedral twins (CELL_NOW) by an approximately 180° rotation aboutthe [100] direction in real space. A total of 4079 frames were collectedwith a scan width of 0.5° in ω; all frames were collected with a 20 sexposure time. Frames were integrated with the Bruker SAINT softwarepackage using the appropriate twin matrix. Data were corrected forabsorption effects (TWINABS) with a transmission range of 0.036-0.110.Final unit-cell dimensions were determined based on the refinement ofthe XYZ-centroids of 9904 reflections with ranges 7°<2θ<61°

The structures of compounds 13β and 13δ were solved in CRYSTALS⁷⁰ usingdirect methods (SIR92) and expanded using Fourier techniques. Diagramswere prepared using Cameron.⁷¹

The coordinates and anisotropic temperature factors for all atoms ofcompounds 13β and 13δ were refined. For 13β, the final refinement usingobserved data (I_(o)>2.50σ(I_(o))) included an extinction parameter, andstatistical weights included 103 parameters for 1656 unique reflections.For 138, the final refinement using observed data (I_(o)>2.50σ(I_(o)))and statistical weights included 104 parameters for 2105 uniquereflections. Selected bond lengths and angles are given in Tables 10 and11 respectively.

X-ray powder diffractograms were collected on a Rigaku RAXIS rapidcurved image plate area detector with a graphite monochromated Cu-Kαradiation source and a 0.5 μm collimator. Powder samples were adhered toa glass fiber with grease. Peak positions for 13γ and cell parameterswere determined with Dicvol.⁷² Structural models for 13γ were producedand refined with Powder Cell⁷³ using triclinic symmetry. The spacegroupand atomic positions of the carbon and nitrogen atoms were determinedwith CRYSTALS⁷⁰ using distance and angle restraints. A final refinementcycle for y was conducted in Powder Cell⁷³ in the tetragonal spacegroupP bar-4 b 2.

The powder pattern of {Zn(NH₃)₂[Au(CN)₂]₂} 14 was compared withsimulated patterns of the coordination polymer {Cd(NH₃)₂[Ag(CN)₂]₂},⁷⁴which was found to have a similar diffraction pattern. Unit celldetermination and further refinements were performed in Powder Cell⁷³using the atomic positions of the {Cd(NH₃)₂[Ag(CN)₂]₂} complex.⁷⁴ Therelative intensities of the 21l reflections for the observed powderpattern were consistently less intense and very broad then theintensities in the calculated powder diffractogram.

2.2.4 Synthesis and IR Spectra

Equations 1-4 below summarize schemes identified by the inventors forcontrolled synthesis of polymorphs 13α-13δ by the reaction of Zn(II)salts with two equivalents of the linear anionic Au(CN)₂ ⁻ buildingblock. All four materials have comparable elemental analyses and thusare true polymorphs, not pseudo-polymorphic solvent adducts. Elementalanalysis indicated that when 13β was initially formed (eq 2), itcontained at least one labile acetonitrile per Zn[Au(CN)₂]₂ unit. Thiscomplex readily desolvated when left in an unsealed container overnight.Powder X-ray diffractograms showed no difference between the solvatedand desolvated forms of 13β.

Polymer 13 exhibits exquisite structural sensitivity to syntheticconditions. More particularly, the synthesis of each polymorph issensitive to solvent choice,⁷⁵⁻⁷⁸ concentration,⁷⁹ pH^(80,81) and eventhe counterions associated with either the zinc(II) or gold(I) startingmaterial, despite the fact that these counterions are not incorporatedinto the final polymer. This last point is best exemplified in thesynthesis of 13βvs. 13γ (Equation 2 & 3), where changing the goldcounterion from K⁺ to [^(n)Bu₄N]⁺ and the zinc counterion from NO₃ ⁻ toClO₄ ⁻ generates 13γ instead of 13β, under similar reaction conditions.Furthermore, if only one counterion is changed, a mixture of polymorphs13β and 13γ are obtained. The counterions in the synthesis of 13β and13γ could play an important role as a templating agent, preferentiallyinducing the formation of one network versus another.

Conversely, the synthetic route to 13α is insensitive to counterions andmoderately insensitive to concentration, although mixtures of 13α and13δ are formed under extremely concentrated conditions (2 mL total).Changing the solvent in equation 1 from water to methanol producescrystals of 13β only when Zn(ClO₄)₂ is used (See experimental sectionabove). The difference may be partially attributed to changes in thehydrogen bonding characteristics of water vs. methanol.⁸² The sameeffect is also observed in the synthesis of 13δ, which is done underacidic conditions (Equation 4) with ZnCl₂.

While the rationale behind the preferential formation of a particularpolymer under a defined set of conditions is unclear, it is obvious(Equation 1-4) that the formation constant for each polymorph isrelatively similar. Changing the solvent, concentration, counterion,and/or pH is sufficient to easily shift the resultant energy minimumfrom one polymorphic form to another.

The IR spectra of all four polymorphs, 13α-13δ, are similar, havingstrong ν_(CN) stretches between 2187 and 2199 cm⁻¹ and readily visible¹³C-satellites between 2158 and 2149 cm⁻¹. These bands are all shiftedto higher energy relative to KAu(CN)₂ (ν_(CN)=2141 cm⁻¹), indicatingthat all of the cyanide groups are bound to a zinc centre.⁴³ For themost part, the similarities in the IR spectra for 13α-13δ precluded theuse of the IR signatures as a definitive polymorph identifier; this wasaccomplished on the basis of a combination of distinct crystal habit(FIG. 11), distinct X-ray powder diffractograms, and emission data (Seebelow).

2.2.5 Crystal Structure of the Polymorphs

Crystal Structure of 13α. The synthesis, with a Zn:Au(CN)₂ ratio of 3:1,and the crystal structure of the resulting hexagonal crystals ofZn[Au(CN)₂]₂ (13α) were previously reported.²⁰ The inventors havedetermined that the stoichiometrically rational reaction of Zn(II) andtwo equivalents of KAu(CN)₂ in water generates the same hexagonalcrystals (Table 10, FIG. 11); the crystal structure is briefly describedbelow for comparative purposes. The crystal structure contains a zinccentre in a tetrahedral geometry, with four N-bound cyanides (Zn—N bondlengths of 1.939 and 1.978 Å), thereby generating a 3-D coordinationpolymer. The network structure of a consists of corner-sharingtetrahedra, analogous to SiO2-quartz (FIG. 12 a);⁸³⁻⁸⁷ each tetrahedronis defined by a gold(I) atom at each vertex and a zinc(II) atom at thecentre. In order to efficiently utilize the large space betweenneighbouring zinc centres of this quartz-type net, the structure issix-fold interpenetrated (FIG. 12 a-right).^(85,86) The interpenetrationis supported via gold-gold bonds of alternating 3.11 and 3.16 Å, forminga 1-D zig-zag chain of Au(CN)₂ ⁻ with an angle of 114.98° (FIG. 12 b). Asimilar structure was reported for Co[Au(CN)₂]₂.¹⁷

Crystal Structure of 13β. Rectangular plate crystals of 13β (FIG. 11)were obtained from the partial evaporation of a methanol solution ofZn(ClO₄)₂ and KAu(CN)₂. Similar to the crystal structure of 13α, 13βalso consists of a zinc(II) centre surrounded by 4 N-bound cyanidegroups in a tetrahedral geometry with Zn—N bond lengths of1.941(14)-1.961(14) Å (Table 11). The tetrahedra are corner-sharing,this time forming a 3-D structure that has a diamond-typetopology;^(82,85,86,88-90) each building block in the network can beviewed as an adamantyl unit (FIG. 3 a). The framework is analogous tocristobalite, another polymorph of SiO₂ (FIG. 3).⁸⁷ The 3-D networks inβ are five-fold interpenetrated (FIG. 13 b-right).^(85,86,89) Theinterpenetrated networks are linked via gold-gold bonds ranging from3.1471(11)-3.2702(6) Å and angles ranging from 104.951(17)-180° (Table11, FIG. 13 c). Whereas the aurophilic array of a forms 1-D chains ofAu-centres, the Au-array in β forms a 2-D (6,3)-network⁸⁵ in which thegold atoms form a distorted hexagonal motif array with gold atomslocated at the vertices of the hexagons (FIG. 13 c).

Crystal Structure of 13γ. Although single crystals of 13γ could not beobtained, pure microcrystalline powder of 13γ was synthesized fromZn(ClO₄)₂ and [cation][Au(CN)₂] (cation=K⁺,^(n)Bu₄N⁺) in MeCN. Thepowder diffractogram of 13γ was observed to be similar to the previouslyreported Pb[Au(CN)₂]₂ structure.¹²⁶ Using this Pb(II) structure as astarting model, the structure of 13γ was determined from powder X-raydiffraction data. An excellent match between predicted and experimentalpowder diffractograms (FIG. 15) was obtained. The structure is bothchemically reasonable and spectroscopically consistent. Interestingly,the crystal structure of 13γ has the same network structure as 13β: adiamond-type array formed by fused adamantyl units (FIGS. 13 a and 13b}).^(82,85,86,88-90) There are several differences between thesepolymorphs. Firstly, the networks of 13γ are four-fold interpenetrating(FIG. 14 b) while 13β contains five independent networks (FIG. 13c).^(85,89) The interpenetration in 13γ is supported by gold-gold bondsof 3.29 Å. However, while a 2-D array of gold atoms is present in β, thegold atoms in 13γ primarily form dimers. Long gold-gold interactions of3.58 Å link the dimers to one another (FIG. 14 b). In addition, theshape of the diamond network in 13γ is more prolate than that of 13β. Asingle adamantyl framework in 13β has the dimensions 25.8×16.5×13 Å(FIG. 3 a) while the adamantyl framework in 13γ has the dimensions33.8×9.6×9.6 Å.

Crystal Structure of 13δ. Twinned cross-shaped crystals of 13δ (FIG. 11)were obtained from an acidic aqueous solution of ZnCl₂, KAu(CN)₂ andKCN; this synthesis was modified from an old report of Zn/Au(CN)₂reactivity that did not identify any of the polymorphs.⁹¹ As in 13α-13γ,the crystal structure of 13δ contains a tetrahedral zinc centre withZn—N(cyano) bond lengths ranging from 1.956(10) to 1.986(10) Å. However,while 13α, 13β, and 13γ form easily recognizable structures based oncorner-sharing tetrahedra, the 3-D structure of 13δ is considerably morecomplicated. Temporarily omitting one of the Au(CN)₂ ⁻ units on eachzinc (the Au(CN)₂ ⁻ unit formed by Au(3)), the structure can besimplified to a corrugated 2-D (6,3)-herringbone structure (FIGS. 16 a &16 b) along the (1 0 −1) plane.^(85,88) A second (6,3)-herringbonenetwork is interwoven through the first (FIG. 16 b-right). Longgold-gold distances of 3.6430(9) Å represent the only close contactbetween this pair of networks. Via the previously omitted Au(CN)₂ ⁻unit, each individual herringbone network is linked to four additionalnetworks (FIG. 16 c-left)—the pair of interwoven networks above, and thepair of networks below—completing the basic 3-D structure. The voidspace is filled by two additional identical 3-D interpenetrated networks(FIG. 16 c-right).⁸⁵ The interpenetration is supported via gold-goldbonds of 3.3318(4) and 3.3382(5) Å (FIG. 16 d). The gold-gold bonds in δare longer then those observed in 13α, 13β, and 13γ.

Polymorphism has been extensively investigated in coordination polymers,with structural differences between polymorphic forms attributed toconnectivity, interpenetration, and degree of solvent inclusion(pseudo-polymorphism).^(58,78,88,92-94) Even the widely investigatedprototypical Prussian Blue-like system, Mn[Fe(CN)₆], has been observedin two polymorphic forms: the standard rock-salt structure, and thedoubly interpenetrated form.⁹⁵

In the case of the four polymorphs 13α-13δ all have a zinc centre in atetrahedral geometry. The difference between the networks of 13α and13δ, and the diamond-like networks of 13β and 13γ is due to the pathwayconnecting zinc centres. Furthermore, the four polymorphs differ in thedegree of interpenetration, decreasing from six to three from 13α to 13δrespectively. In all of the polymers the interpenetration is supportedby gold-gold bonds ranging from 3.11-3.34 Å. While it is generallybelieved that a lower degree of interpenetration is ideal for creatingempty cavities,⁸² it is interesting to note that 13β has a significantlylower density then the other polymorphs, despite the relatively highdegree of interpenetration.^(48,96,97)

2.2.6 Thermal Stability

All four polymorphs are stable until at least 350° C., after which theybegin to decompose, losing all of the cyanide groups in one step between350-390° C. Thus, despite the different levels of interpenetration andsupporting gold-gold networks, the four polymorphs show very similarthermal stabilities. In addition, DSC measurements of 13α-13δ over therange 25-300° C. show no indication of a phase change from one polymorphto another.

2.2.7 Photoluminesence

Gold (I) centres which are separated by less then 3.6 Å (the sum of theVan der Waals radii for gold)⁹⁸ are said to show gold-gold (aurophilic)bonding;⁹⁹ such systems are known to be potentially luminescent. As aresult of their interesting emission properties, compounds containinggold-gold bonds have received a great deal of attention.¹⁰⁰⁻¹⁰³ In thecrystal structures of 13α-13δ, network interpenetration is supported viagold-gold bonds on the order of 3.11-3.34 Å and indeed, 13α-13γ areemissive when exposed to UV light at room temperature.

The photoluminescence spectra of 13α-13δ are summarized in Table 12.Compound 13α shows two emission bands at 390 and 480 nm (FIG. 17) atroom temperature. The excitation spectra show an identical excitationmaximum at 345 nm for both emission bands (FIG. 17 bottom). However, forcrystals, or densely packed powder samples of 13α, the 480 nm emissionband can be directly excited at 390 nm with a concomitant change in therelative intensities of the 390 and 480 nm emissions. The lifetimes ofboth emission energies were measured in order to determine the nature ofthe emission. The 390 nm emission has a lifetime of 240 ns while the 480nm emission has a lifetime of 930 ns. Based on this data and thelifetimes of other Au(CN)₂ ⁻-based coordination polymers,^(104,105) the390 and 480 nm emissions are assigned to a singlet (flouresence) andtriplet (phosphoresence) emission respectively. Due to the largespin-orbit coupling of gold, phosphoresence is generally the predominantemission pathway. The presence of both types of emission, as in 13α, isless common.¹⁰⁵

In contrast, 13β and 13γ show only one emission band with similarenergies at 450 and 440 nm respectively (Table 12, FIG. 17). For 13δ,despite the presence of gold-gold bonds, hand-picked single crystals ofδ showed no observable room temperature luminescence. The absorptionspectra of 13α-13γ showed that the lowest energy absorption band isconsistent with the lowest fluorescence excitation energy, confirmingthat the observed emissions are attributed to the polymorphs. OtherAu(CN)₂ ⁻-based coordination polymers have also shown excitation andemission energies in this range.100,103,106

It has been observed both theoretically and experimentally that thegold-gold distances in a structurally related series of metal-metalbonded gold(I) systems are inversely proportional to the emissionenergies. Indeed, the low energy, phosphoresence, emissions of 13α-13γobey this trend; on average, shorter gold-gold bonds in α (3.11 and 3.16Å) emit at lower energy than the gold-gold bonds in 13β(3.1471(11)-3.2702(6) Å) and 13γ (3.29 Å). A plot of average Au—Audistance vs. emission energy yields a straight line and also predictsthat 13δ should emit at around 427 nm. However, a non-emissive decaypathway may be more prevalent in 13δ at room temperature, rendering ittruly non-emissive.

2.2.8 Response to Ammonia Exposure

Based on the sensitive and dramatic visible and IR spectral response ofthe related Cu[Au(CN)₂]₂(H₂O)₂ system to ammonia gas as described above,the response of the colourless polymorphs 13α-13δ to ammonia vapour wasexamined, with particular focus on possible emission changes acting as asensory output. Ammonia detectors have a variety of applications inagriculture, the automotive industry, industrial refrigeration, medicaldiagnosis, and anti-terrorism.¹⁰⁷ From a health perspective, the humannose is capable of sensing ammonia at a concentration of 50 ppm, but thepermissible long term exposure limit of ammonia is below this, at 20ppm.^(107,108) For these reasons, the design of materials for thedetection of ammonia is of great interest.

When the four polymorphs 13α-13δ were exposed to a large excess of NH₃gas, a new white powder was formed, the IR spectrum of which containedonly one ν_(CN) band at 2141 cm⁻¹, indicative of free Au(CN)₂ ⁻ units.The IR spectrum also showed the presence of metal-bound ammonia,¹⁰⁹suggesting that the zinc(II) centre is either tetrahedral, coordinatedby four ammonia units, or octahedral, coordinated by six ammonia units;the former is more likely on the basis of typical Zn(II)-amminecoordination chemistry¹¹⁰⁻¹¹² and thus this ammonia-saturated powder istentatively formulated as {Zn(NH₃)₄[Au(CN)₂]₂} 15. The room temperatureluminescence spectrum of this ammonia-saturated complex shows a singleemission peak at 430 nm with an excitation band at 365 nm (FIG. 19).Unfortunately, rapid loss (seconds) of some of the ammonia occurs whenthe sample is unsealed, precluding further structural or elementalanalysis.

Removal from the ammonia-rich atmosphere and subsequent rapid loss ofexcess ammonia from {Zn(NH₃)₄[Au(CN)₂]₂} 15 generates a yellow powder(FIG. 18), which is stable in the absence of ammonia for 30 minutes.Elemental analysis of this yellow powder is consistent with abis-ammonia adduct of Zn[Au(CN)₂]₂13, namely {Zn(NH₃)₂[Au(CN)₂]₂}14. TheIR spectrum of {Zn(NH₃)₂[Au(CN)₂]₂}14 shows a single ν_(CN) band at 2158cm⁻, suggesting that all of the Au(CN)₂ ⁻ units occupy a similarenvironment. Furthermore, the vibrational modes associated with theammonia molecule suggest a metal-bound ammine is present.¹⁰⁹ Comparisonof the powder X-ray diffractogram of {Zn(NH₃)₂[Au(CN)₂]₂} 14 to otherclosely related bis-ammonia coordination polymers¹¹³ reveals a similardiffractogram to both {Cd(NH₃)₂[Ag(CN)₂]₂}⁷⁴ and{Cu(NH₃)₂[Ag(CN)₂]₂}.¹¹⁴ The powder pattern of the cadmium polymer is abetter fit, which is consistent with the fact that both zinc(II) andcadmium(II) do not contain the Jahn-Teller axis present in thecopper(II) system. Using the Cd-system as a starting model, thesolid-state structure of {Zn(NH₃)₂[Au(CN)₂]₂} was determined by fittingatomic coordinates to the experimental powder X-ray diffraction pattern.A good match was obtained (FIG. 20), except for the {21l} reflections.These planes intersect Zn atoms, making them sensitive to NH₃ gain/loss,and are thereby much broader in comparison with the remaining peaks.Consistent with the IR data, the structure contains an octahedral zinccentre in D_(4h) geometry having trans-ammonia molecules and fourN-bound cyanides. Linking through the Au(CN)₂ ⁻ units, the zinc centresform a 2-D corrugated sheet (FIG. 21 a),^(85,86) with Au(CN)₂ ⁻ units atthe apex of the corrugation.⁷⁴ Additional parallel sheets stack viagold-gold bonds of 3.06 Å (FIG. 21 b-left). Rather then forming chainsor sheets of gold-gold bonds as in 13α, 13β and 13δ, only discretedimers of Au(CN)₂ ⁻ units are found, similar to 13γ. A second set ofsheets are inclined and interpenetrated through the aforementionedparallel sheets.⁸⁵ These perpendicular sheets show no close gold-goldbonds or other interactions to one another. Room temperaturephotoluminescence of {Zn(NH₃)₂[Au(CN)₂]₂} 14 shows a single emissionband at 500 nm with an excitation at 400 nm (FIG. 19), which alsomatches the Au—Au distance/energy correlation graph.

The TGA of {Zn(NH₃)₂[Au(CN)₂]₂} 14 shows a drop starting at roomtemperature and ending at 95° C. The mass loss is consistent with a lossof two equivalents of ammonia (% loss observed 6%; calculated 5.7%). Asthe temperature is increased a second loss from 350 to 390° C. isobserved, consistent with decomposition of the polymer via cyanide lossand formation of ZnO and Au (% loss observed 15.2%; calculated 14.7%),as seen for 13α-13δ. However, this ammonia loss also occurs withoutheating: if {Zn(NH₃)₂[Au(CN)₂]₂} 14 is left open to air for 30 minutesall ammonia molecules are released, leaving behind a white powder ofZn[Au(CN)₂]₂13. Interestingly, the X-ray powder diffractogram of thisammonia-free product indicated that a mixture of both 13α and 13δpolymorphs had been generated. However, the ratio of the two polymorphsis sample-dependent; cases ranging from pure 13α to pure 13δ and variousratios in between have all been observed (the controlling factor remainsunclear). Photoluminescence spectra of the mixtures is consistent withthe luminescence of 13α, and mixtures of 13α and non-emissive 13δ. It isimportant to note that reintroduction of ammonia vapour to any mixtureof 13α:13δ, or even to pure δ, invariably regenerates the saturated{Zn(NH₃)₄[Au(CN)₂]₂} 15 complex, followed by the bis-ammonia complex{Zn(NH₃)₂[Au(CN)₂]₂} 14 (as confirmed by powder X-ray diffractionmeasurements in situ) upon removal of the ammonia-rich atmosphere.

In order to probe the mechanism of adsorption/desorption:¹¹⁵ titrationof all four polymorphs with sequential equivalents of NH₃, monitoringthe ν_(CN) stretch in the IR, was investigated. Titration of 13α and 13δreveals that they bind ammonia in a stepwise fashion (Equation 5), firstconverting to {Zn(NH₃)₂[Au(CN)₂]₂} 14 (ν_(CN) 2158 cm⁻¹) and then to{Zn(NH₃)₄[Au(CN)₂]₂} 15 (ν_(CN) 2141 cm⁻¹). This reaction is completelyreversible; i.e., loss of ammonia regenerates the same polymorph.However, titration of 13β and 13γ reveals that the tetraaminezinc(II)complex forms directly (Equation 6), with no intermediate of{Zn(NH₃)₂[Au(CN)₂]₂} observed. Apparently, sufficient ammonia must bepresent to break all the bonds between the zinc(II) and the fourcyanides in order for initial NH₃-uptake to occur. Due to the adsorptionroutes for the four polymorphs, it is clear that once{Zn(NH₃)₄[Au(CN)₂]₂} is formed from 13D and 13γ, these polymorphs cannotbe regenerated upon NH₃-desorption; mixtures of 13α and 13δ are producedinstead.

(α/δ)Zn[Au(CN)₂]₂+2NH₃ _((g)) →{Zn(NH₃)₂[Au(CN)₂]₂}

{Zn(NH₃)₂[Au(CN)₂]₂+2NH₃ _((g)) →{Zn(NH₃)₄[Au(CN)₂]₂}  (5)

(β/γ)Zn[Au(CN)₂]₂+4NH₃ _((g)) →{Zn(NH₃)₄[Au(CN)₂]₂}

{Zn(NH₃)₄[Au(CN)₂]₂}→{Zn(NH₃)₂[Au(CN)₂]₂}+2NH₃ _((g))   (6)

Porosity measurements on all four polymorphs showed no adsorption ofnitrogen gas at 77 K. Despite this, 13α-13δ reversibly bind ammoniavapour. It is well-known that non-porous coordination polymers can bequite flexible in the solid-state.^(54,116,117) The d¹⁰ zinc(II) cationcan adopt several geometries encompassing coordination numbers from twoto six;^(102,118) undoubtedly, this flexibility facilitates thestructural rearrangement which occurs as the ammonia interacts with thezinc(II) centre. Note that although NH₃-binding occurs at Zn(II), it isthe array of Au(CN)₂ ⁻ units that act as the sensor, as the structuralrearrangement changes the gold-gold distance and thus the emissionenergy.

As discussed above, ammonia sensors are used in a wide range ofsettings.¹⁰⁷ For each application there are different requirements foran ideal sensor, including detection limits (0.1 ppb-200 ppm), responsetimes (seconds-minutes), and operating temperatures (0-500° C.).¹⁰⁷ Dueto the highly varying requirements there are several different types ofsensors employed in the detection of ammonia. Some common ammonia-sensormaterials include conducting compounds based on metal-oxides¹¹⁹⁻¹²¹ orpolymers such as polypyrrole¹²² and polyaniline;¹²³ other sensors arebased on spectrophotometric determination.^(124,125) While metaloxide-based sensors are very common, they require a high operatingtemperature, making them unsuitable in some applications, e.g. medicaldiagnostics.¹⁰⁷ Conversely, although atomic absorption-based sensing canbe operated at room temperature with high sensitivity, it is extremelyexpensive.¹⁰⁷

This example clearly shows that all four polymorphs of zinc polymer 13act as vapoluminescent sensors for ammonia. However, in order todetermine if the Zn[Au(CN)₂]₂ materials (which are relativelyinexpensive and quite stable) could compete with current NH₃-detectionsystems, quantitative detection limits for each polymorph were measured,by monitoring the intensity of the {Zn(NH₃)₂[Au(CN)₂]₂} emission band(λ_(max)=500 nm, λ_(ex)=400 nm) emission band at 520 nm. Althoughsensitivities of the polymers vary depending on polymorph, the lowestdetection limit was observed for 13β, with a NH₃-detection limit of 1ppb. The response times are also fast: within seconds for allpolymorphs. In comparison with other materials used as ammonia sensors,these four coordination polymers have some of the lowest NH₃-detectionlimits and response times.

In summary, the reaction of various zinc salts with the linear, anionicAu(CN)₂ ⁻ bridging ligand formed four structurally unique polymorphs,three of which are luminescent at room temperature. Care must be takenwhen synthesizing these polymorphs since changing the concentration,counterion, pH, and/or solvent can redirect the synthesis from onepolymorphic form to another. All four polymorphs reversibly act as verysensitive sensors for ammonia vapour, changing their emission energiesas ammonia is bound. The emission can be correlated to the gold-goldarray distance in each compound.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. Accordingly, the scope of the invention is to beconstrued in accordance with the substance defined by the followingclaims.

TABLE 1 Crystallographic Data and Structural Refinement Details 1(Green) 2 (Blue) 3 4 empirical C₈H₁₂N₄Au₂CuO₂S₂ C₈H₁₂N₄Au₂CuO₂S₂C₇H₇N₅Au₂CuO C₁₄H₁₀N₆Au₂Cu formula fw 717.82 717.82 634.65 719.76Crystal system monoclinic triclinic monoclinic monoclinic Space groupC2/c P 1 C2/c P2₁/c a, Å 11.5449(15) 7.874(7) 12.8412(10) 7.3438(7) b, Å14.191(4) 12.761(11) 14.5056(8) 14.1201(10) c, Å 11.5895(12) 16.207(13)13.9932(9) 8.2696(6) α, deg 90 89.61(7) 90 90 β, deg 112.536(9) 82.29(7)96.064(3) 94.082(3) γ, deg 90 88.57(7) 90 90 V, Å³ 1753.8(6) 1613.2(24)2591.9(3) 855.34(12) Z 4 2 8 4 T, K 293 293 293 293 λ, Å 0.70930 1.541801.54180 1.54180 ρ_(calcd), g · cm⁻³ 2.719 2.955 3.253 2.794 μ, mm⁻¹18.079 37.500 43.542 33.103 R₁ ^(a) (I > xσ(I))^(b) 0.042 0.062 0.0320.028 wR₂ ^(a) (I > xσ(I))^(b) 0.047 0.082 0.046 0.040 Goodness of fit2.20 1.38 0.93 1.00 ^(a)Function minimized Σw(|F_(o)| − |F_(c)|)² wherew⁻¹ = σ²(F_(o)) + 0.0001F_(o) ², R = Σ||F_(o)| − |F_(c)||/Σ|F_(o)|,R_(w) = [Σw|F_(o)| − |F_(c)|)²/Σw|F_(o)|²)^(1/2). ^(b)For 1, x = 2.5;for 2, 3 and 4, x = 3.

TABLE 2 Selected bond lengths (Å) and angles (°) for Cu[Au(CN)₂]₂(DMSO)₂(1). Au(1)—Au(2) 3.22007(5) Cu(1)—O(1) 1.949(7) O(1)—Cu(1)—O(1*)167.0(6) O(1)—Cu(1)—N(2) 96.5(3) O(1)—Cu(1)—N(3) 87.3(4)O(1*)—Cu(1)—N(3) 88.4(4) N(2)—Cu(1)—N(3) 109.5(4) N(3)—Cu(1)—N(3*)140.9(8) Cu(1)—O(1)—S(1) 127.2(6) Cu(1)—N(2) 2.107(18) Cu(1)—N(3)1.965(11) Cu(1)—N(2)—C(2) 180 Cu(1)—N(3)—C(3) 178.6(12)Au(2)—Au(1)—Au(2) 171.73(3) Au(1)—N(1)—C(1) 180 Au(1)—N(2)—C(2) 180Au(2)—N(3)—C(3) 178.9(12) C(1)—Au(1)—C(2) 180 Symmetry transformations:(*) −x + 1, y, −z + ½; (′) −x + ½, −y − 5/2, z + 1.

TABLE 3 Selected bond lengths (Å) and angles (°) for Cu[Au(CN)₂]₂(DMSO)₂(2) Au(1)—Au(2) 3.419(3) Cu(1)—O(1) 2.02(3) Cu(1)—O(2) 1.95(3)Cu(1)—N(11) 2.42(4) Cu(1)—N(21) 1.97(4) Cu(1)—N(31) 2.42(4) Cu(1)—N(41)1.99(4) O(1)—Cu(1)—O(2) 95.2(12) O(1)—Cu(1)—N(11) 85.9(12)O(2)—Cu(1)—N(11) 86.4(12) N(11)—Cu(2)—N(21) 92.7(14) N(11)—Cu(2)—N(31)172.7(13) N(11)—Cu(2)—N(41) 92.6(14) N(21)—Cu(2)—N(31) 92.6(15)N(21)—Cu(2)—N(41) 90.7(15) N(31)—Cu(2)—N(41) 92.3(14) Cu(1)—O(1)—S(1)124.9(17) Cu(1)—O(2)—S(2) 124.4(19) Cu(1)—N(11)—C(11) 169.2(45)Cu(1)—N(21)—C(21) 163.5(41) Cu(1)—N(31)—C(31) 161.7(43)Cu(1)—N(41)—C(41) 166.4(33) C(11)—Au(1)—C(12) 172.7(25)C(21)—Au(2)—C(22*) 175.9(23) Au(1)—C(11)—N(11) 175.8(50)Au(1)—C(12)—N(12) 175.3(58) Au(2)—C(21)—N(21) 173.2(42)Au(2*)—C(22)—N(22) 175.2(56) Au(3) . . . Au(4) 3.592(4) Cu(2)—O(3)1.97(3) Cu(2)—O(4) 2.29(3) Cu(2)—N(12) 2.11(4) Cu(2)—N(22) 2.37(5)Cu(2)—N(32) 2.03(5) Cu(2)—N(42) 2.00(5) O(3)—Cu(2)—O(4) 93.0(12)O(3)—Cu(2)—N(12) 87.8(15) O(4)—Cu(2)—N(12) 87.0(14) N(12)—Cu(2)—N(22)92.3(16) N(12)—Cu(2)—N(32) 172.0(17) N(12)—Cu(2)—N(42) 95.2(17)N(22)—Cu(2)—N(32) 91.4(17) N(22)—Cu(2)—N(42) 91.2(17) N(32)—Cu(2)—N(42)91.8(18) Cu(2)—O(3)—S(3) 125.4(20) Cu(2)—O(4)—S(4) 127.9(18)Cu(2)—N(12)—C(12) 163.5(50) Cu(2)—N(22)—C(22) 159.5(46)Cu(2)—N(32)—C(32′) 174.6(45) Cu(2)—N(42)—C(42) 170.0(45)C(31)—Au(3)—C(32) 172.6(18) C(41*^(b))—Au(4)—C(42) 177.9(20)Au(3)—C(31)—N(31) 171.0(39) Au(3)—C(32)—N(32′^(b)) 175.6(49)Au(4*^(b))—C(41)—N(41) 174.2(38) Au(4)—C(42)—N(42) 170.1(46) Symmetrytransformations: (*) −x + 1, −y + 1, −z + 1; (*^(b)) −x + 1, −y, −z + 1;(′) x + 2, y, z − 1; (′^(b)) x − 2, y, z + 1.

TABLE 4 Selected bond lengths (Å) and angles (°) for Cu[Au(CN)₂]₂(DMF)(3). Au(1)—Au(1*^(a)) 3.3050(12) Au(2)—Au(2*^(b)) 3.1335(13) Cu(1)—O(1)2.202(12) Cu(1)—N(1′) 1.958(10) N(1′^(a))—Cu(1)—N(2) 89.8(4)N(1′^(a))—Cu(1)—N(3) 88.7(5) N(4′^(b))—Cu(1)—N(2) 89.6(5)N(4′^(b))—Cu(1)—N(3) 89.3(5) N(1′^(a))—Cu(1)—N(4′^(b)) 166.7(5)N(2)—Cu(1)—N(3) 169.2(5) O(1)—Cu(1)—N(1′^(a)) 95.1(5) O(1)—Cu(1)—N(2)98.3(5) O(1)—Cu(1)—N(3) 92.4(5) O(1)—Cu(1)—N(4′^(b)) 98.1(5)Cu(1)—O(1)—C(5) 125.4(13) Cu(1)—N(2) 1.990(11) Cu(1)—N(3) 1.961(10)Cu(1)—N(4′^(b)) 1.982(10) O(1)—C(5) 1.202(17) C(1)—Au(1)—C(2) 176.0(6)C(3)—Au(2)—C(4) 175.4(6) Cu(1′^(c))—N(1)—C(1) 170.1(12) Cu(1)—N(2)—C(2)172.7(14) Cu(1)—N(3)—C(3) 170.8(12) Cu(1′^(d))—N(4)—C(4) 172.1(12)Au(1)—C(1)—N(1) 174.9(13) Au(1)—C(2)—N(2) 177.8(14) Au(2)—C(3)—N(3)174.3(13) Au(2)—C(4)—N(4) 177.5(16) Symmetry transformations: (*^(a)) −x− 1, y, −z + 3/2; (*^(b)) −x − 1, y, −z + ½; (′^(a)) x, −y, z − ½;(′^(b)) x, −y − 1, z + ½; (′^(c)) x, −y, z + ½; (′^(d)) x, −y − 1, z −½.

TABLE 5 Selected bond lengths (Å) and angles (°) forCu[Au(CN)₂]₂(pyridine)₂ (4). Cu(1)—N(1) 2.016(9) Cu(1)—N(2*^(a))2.532(9) N(1)—Cu(1)—N(2*^(a)) 89.5(4) N(1′)—Cu(1)—N(2*^(a)) 90.5(4)N(1)—Cu(1)—N(3) 90.0(3) N(1)—Cu(1)—N(3′) 90.0(3) N(2*^(a))—Cu(1)—N(3)90.4(3) N(2*^(a))—Cu(1)—N(3′) 89.6(3) Cu(1)—N(3) 2.007(7)C(2)—Au(1)—C(1) 177.8(4) Cu(1)—N(1)—C(1) 169.7(9) Cu(1*^(b))—N(2)—C(2)173.3(9) Au(1)—C(1)—N(1) 177.9(9) Au(1)—C(2)—N(2) 177.2(11) Symmetrytransformations: (*^(a)) x − 1, −y + ½, z − ½; (*^(b)) x + 1, −y + ½,z + ½; (′) −x + 1, −y, −z + 1.

TABLE 6 Maximum Solid-state Visible Reflectance (nm) and Cyanide ν_(CN)Absorptions (cm⁻¹) for Different Cu[Au(CN)₂]₂(solvent)_(x) ComplexesMaximum visible ν_(CN) absorption(s) Complex reflectance From solutionFrom adsorption^(a) (1) Cu[Au(CN)₂]₂(DMSO)₂ 550 ± 7 2183(s), 2151(s)2184(s), 2151(s) (from 5) (2) Cu[Au(CN)₂]₂(DMSO)₂ 535 ± 15 2206(m),2193(s), — (broad) 2175(m), 2162(m) (3) Cu[Au(CN)₂]₂(DMF) 498 ± 72199(s) 2199 (s) (4) Cu[Au(CN)₂]₂(pyridine)₂ 480 ± 15 2179(m), 2167(s),2179(m), 2167(s), 2152(m), (broad) 2152(m), 2144(m) 2144(m) (5)Cu[Au(CN)₂]₂(H₂O)₂ 535 ± 5 2217(s), 2194(w), 2217(s), 2194(w), 2171(s)2172(s) (from 1) 2216(s), 2196(w), 2171(s) (from 2) (6) Cu[Au(CN)₂]₂ 560± 20 2191(s) — (v. broad) (7) Cu[Au(CN)₂]₂(CH₃CN)₂ — 2297(w), 2269(w),2191(s) (8) Cu[Au(CN)₂]₂(dioxane)(H₂O) 505 ± 15 2201(s), 2172(w)2200(s), 2174 (w) (broad) (9) Cu[Au(CN)₂]₂(NH₃)₄ 433 ± 7 — 2175(m),2148(s) ^(a)All solvent adducts were made from 2 unless specified

TABLE 7 Thermal Decomposition of Different Cu[Au(CN)₂]₂(solvent)_(x)Complexes Temperature Decomposition product Weight (%) Complex (° C.) orlost fragment calcd found 3 195-280 —DMF 11.5 13.6 310-355 —2C₂N₂ + O13.8 11.5 400 CuO + 2Au 74.6 73.9 4 155-190 —1 pyridine 11.0 10.9210-260 —1 pyridine 11.0 12.6 310-330 —2C₂N₂ + O 12.2 9.2 400 CuO + 2Au65.8 66.4 5 140-180 —2 water 6.0 5.5 260-380 —2C₂N₂ + O 14.7 13.5 400CuO + 2Au 79.2 81.5 6 200-350 —2C₂N₂ + O 15.2 15.5 400 CuO + 2Au 81.780.9 8 150-280 —dioxane-H₂O 15.9 17.5 290-330 —2C₂N₂ + O 13.2 10.3 400CuO + 2Au 70.9 71.1 9 50-95 —1NH₃ 2.7 2.8 115-220 —3NH₃ 8.1 7.5 280-350—2C₂N₂ + O 14.0 13.7 400 CuO + 2Au 75.2 74.4

TABLE 8 List of Selected VOCs that 1 can Detect, Spectral Data, and ItsSensitivity Minimum Minimum v_(CN) for 1 detectable λ_(max) v_(CN) for 1λ_(max) Response detectable (VOC) at concentration (nm) at Reversible(solvent)/ (nm)/ time at concentration sensitivity by visiblesensitivity without VOC excess excess saturation by IR limit reflectancelimit Heating Compound 1 2216, 532 N/A N/A N/A N/A N/A N/A 2171 Ammonia2141, 435 <5 s 36 ppb 2181, 230 ppb 510 No 2136 2151 Methylamine 2139438 <8 s 120 ppb 2188 360 ppb 495 No 2139 Propylamine 2143 428 <10 s 140ppb 2185, 320 ppb 502 No 2155 Butylamine 2141 460 2 min 100 ppm 2141 860ppm 468 No Diethylamine 2144 478 10 min 600 ppm 2144 1600 ppm 483 NoTrimethylamine Not Not N/A N/A N/A N/A Not N/A sensitive sensitivesensitive Ethylenediamine 2141 408 10 s 385 ppb 2141 720 ppb 415 NoPyridine 2140, 485 8 min 400 ppm 2193, 875 ppm 490 Yes 2133 2148 2142Acetonitrile 2297, Broad 10 min 470 ppm 2304, 1120 ppm Broad Yes 2269,2266, 2192 2192 DMF 2199, 482 12 min 450 ppm 2199, 1155 ppm 488 Yes 21712171 DMSO 2194, 485 13 min 1000 ppm 2194, 1640 ppm 485 Yes 2176, 2176,2162 2162

TABLE 9 Crystallographic Data for 13α-13δ and {Zn(NH₃)₂[Au(CN)₂]₂} 14α^(a) β γ^(b) δ {Zn(NH₃)₂[Au(CN)₂]₂}^(b) empirical formula C₄N₄Au₂ZnC₄N₄Au₂Zn C₄N₄Au₂Zn C₄N₄Au₂Zn C₄H₆N₆Au₂Zn formula weight 563.40 563.40563.40 563.40 597.45 crystal system hexagonal monoclinic tetragonalmonoclinic tetragonal space group P6₄2₂ C2/c P 4b2 C2/c P4₂/mbc crystalhabit hexagons plates — crosses — a (Å) 8.4520(10) 8.90060(10) 6.82089.9583(3) 7.8554 b (Å) 8.4520(10) 16.8152(2) 6.8208 10.4988(3) 7.8554 c(Å) 20.621(11) 14.3808(2) 8.4487 15.7961(4) 17.0937 α (deg) 90 90 90 9090 β (deg) 90 100.6000(10) 90 98.407(2) 90 γ (deg) 120 90 90 90 90 V(Å³) 1275.8(7) 2115.58(5) 393.03 1633.74(8) 1054.81 Z 6 8 2 8 4 T (K)293 293 293 293 293 ρ_(calcd) (g cm⁻³) 4.400 3.537 4.760 4.581 3.724 μ(mm⁻¹) 37.146 29.868 40.193 38.677 29.967 R [I_(o) ≧ 2.50σ(I_(o))]^(c)0.0459 0.0429 0.0082 0.0338 0.0366 R_(w) [I_(o) ≧ 2.50σ(I_(o))]^(c)0.0616 0.0564 0.0117 0.0543 0.0691 goodness of fit — 1.1438 — 1.2462 —^(a)From ref 50. ^(b)From X-ray powder diffraction data. ^(c)Thefunction minimized was Σw(|F_(o)| − |F_(c)|)², where w⁻¹ = [σ²(F_(o)) +(nF_(o))²] with n = 0 for β, 0.030 for δ, and 0 for γ and{Zn(NH₃)₂[Au(CN)₂]₂. R = Σ||F_(o)| − |F_(c)||/Σ|F_(o)|I; R_(w) =[Σw(|F_(o)| − |F_(o)|)²/Σw|F_(o)|²]^(1/2).

TABLE 10 Bond Lengths (Å) and Angles (deg) for 13β^(a) Zn(1)—N(11)1.957(14) Zn(1)—N(22) 1.955(14) Au(1)—Au(2*) 3.2702(6) Au(2′)—Au(3)3.1925(7) Au(2″)—Au(2^(†)) 3.1466(11) Zn(1)—N(11)—C(11) 168.6(16)Zn(1)—N(22)—C(22) 162.9(14) N(11)—Zn(1)—N(21) 111.4(6) N(11)—Zn(1)—N(31)106.3(6) N(21)—Zn(1)—N(31) 108.9(6) Au(2*)—Au(1)—Au(2^(†)) 180.00Au(3)—Au(2″)—Au(2^(†)) 145.15(3) Au(2′)—Au(3)—Au(2″) 141.53(4)Zn(1)—N(21) 1.944(15) Zn(1)—N(31) 1.964(13) Au(1)—Au(2^(†)) 3.2702(6)Au(2″)—Au(3) 3.1925(7) Zn(1)—N(21)—C(21) 171.5(13) Zn(1)—N(31)—C(31)166.6(14) N(11)—Zn(1)—N(22) 106.4(6) N(21)—Zn(1)—N(22) 112.8(6)N(22)—Zn(1)—N(31) 110.8(6) Au(3^(‡))—Au(2^(†))—Au(1) 104.944(17)Au(1)—Au(2^(†))—Au(2″) 109.35(2) ^(a)Symmetry operations: (*) x + ½, y +½, z; (′) −x + 1, −y + 1, −z; (^(†)) −x + 2, −y + 1, −z; (^(‡)) x + 1,y, z; (″) x, −y + 1, z − ½.

TABLE 11 Bond Lengths (Å) and Angles (deg) for 13δ^(a) Zn(1)—N(11)1.975(10) Zn(1)—N(31) 1.968(10) Au(1)—Au(3′) 3.3382(5) Au(2*)—Au(3″)3.3318(4) N(11)—Zn(1)—N(32) 110.3(4) N(11)—Zn(1)—N(31) 100.6(4)N(21)—Zn(1)—N(32) 101.2(4) Zn(1)—N(11)—C(11) 161.8(10) Zn(1)—N(31)—C(31)158.7(10) Au(3′)—Au(1)—Au(3″) 180.00 Au(1)—Au(3″)—Au(2*) 65.690(8)Zn(1)—N(21) 1.956(10) Zn(1)—N(32) 1.956(10) Au(1)—Au(3″) 3.3382(5)Au(2*)—Au(3) 3.3318(4) N(11)—Zn(1)—N(21) 113.8(4) N(21)—Zn(1)—N(31)118.9(4) N(31)—Zn(1)—N(33) 112.3(5) Zn(1)—N(21)—C(21) 163.1(10)Zn(1)—N(32)—C(32) 159.3(10) Au(3)—Au(2*)—Au(3″) 180.00 ^(a)Symmetryoperations: (′) x − ½, y + ½, z; (″) −x + ½, −y + 3/2, −z + 1; (*) x,−y + 2, z + ½.

TABLE 12 Summary of Luminescence data for 13α-13δ, [Zn(NH₃)₄][Au(CN)₂]₂15, and {Zn(NH₃)₂[Au(CN)₂]₂} 14 emission compound maximum (nm)excitation maximum (nm) α 390, 480 345 β 450 390 γ 440 360 δ none none[Zn(NH₃)₄][Au(CN)₂]₂ 430 365 {Zn(NH₃)₂[Au(CN)₂]₂} 500 400

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1. A vapochromic polymer comprising:M_(W)[M′_(X)(Z)_(Y)]_(N) wherein M and M′ are the same or differentmetals capable of forming a coordinate complex with the Z moiety; Z isselected from the group consisting of halides, pseudohalides, thiolates,alkoxides and amides; W is between 1-6; X and Y between 1-9; and N isbetween 1-5.
 2. The polymer as defined in claim 1, wherein W and X are1, Y is 2, and N is 2 or
 3. 3. The polymer as defined in claim 2,wherein said polymer is reversibly vapochromic.
 4. The polymer asdefined in claim 1, wherein M is a transition metal.
 5. The polymer asdefined in claim 1, wherein M′ is a metal selected from the groupconsisting of Au, Ag, Hg and Cu.
 6. The polymer as defined in claim 1,wherein Z is a pseudohalide selected from the group consisting of CN,SCN, SeCN, TeCN, OCN, CNO and NNN.
 7. The polymer as defined in claim 1,wherein M is selected from the group consisting of Cu, Zn, Sc, Ti, V,Cr, Mn, Fe, Co and a lanthanide.
 8. The polymer as defined in claim 7,wherein M is Cu or Zn.
 9. The polymer as defined in claim 1, wherein M′is selected from the group consisting of Au and Ag and wherein Z is CN.10. The polymer as defined in claim 9, wherein M′ is Au and wherein X is1 and Y is
 2. 11. The polymer as defined in claim 1, further comprisingan organic ligand bound to M.
 12. The polymer as defined in claim 11,wherein said organic ligand comprises at least one nitrogen, oxygen,sulphur or phosphorus donor.
 13. The polymer as defined in claim 1,further comprising a counterbalancing cation or anion.
 14. The polymeras defined in claim 1, wherein said polymer is vapoluminescent.
 15. Thepolymer as defined in claim 1, wherein X=1 and Y=2.
 16. A compositioncomprising a polymer as defined in claim 1, and an analyte adsorbed tosaid polymer.
 17. The composition as defined in claim 16, wherein saidanalyte is a volatile organic compound.
 18. The composition as definedin claim 16, wherein said analyte is a gas having a donor hydrogen,nitrogen, oxygen, sulphur or phosphorus atom.
 19. A solid-statevapochromic structure comprising a polymer as defined in claim
 1. 20. Asolid-state vapochromic structure comprising a composition as defined inclaim
 16. 21. The use of a structure a defined in claim 19 forvapochromically sensing the presence of an analyte.
 22. An analyte-boundcomplex comprising an analyte adsorbed to a polymer as defined inclaim
 1. 23. A method of detecting an analyte comprising: (a) providinga polymer as defined in claim 1; (b) exposing said polymer to a supplyof said analyte; and (c) detecting any chromatic changes in said polymerresulting from exposure to said analyte.
 24. The method as defined inclaim 23, wherein said detecting comprises sensing any changes in thecolour of said polymer.
 25. The method as defined in claim 23, whereinsaid detecting comprises sensing any changes in the luminescence of saidpolymer.
 26. The method as defined in claim 23, wherein said detectingcomprises spectroscopically identifying any changes in the infraredsignature of said polymer.
 27. The method as defined in claim 26,wherein said spectroscopically identifying comprises detecting thenumber and position of ν_(CN) spectroscopic bands.
 28. The method asdefined in claim 23, wherein said chromatic changes are reversible. 29.The method as defined in claim 28, wherein said supply comprisesdifferent analytes and wherein steps (b) and (c) are dynamicallyrepeated in respect of successive ones of said analytes.
 30. The methodas defined in claim 23, wherein M is Cu or Zn and wherein M′ is Au. 31.The method as defined in claim 30, wherein said polymer is selected fromthe group consisting of Cu[Au(CN)₂]₂ and Zn[Au(CN)₂]₂.
 32. The method asdefined in claim 23, wherein said analyte is a volatile organiccompound.
 33. The method as defined in claim 23, wherein said analyte isa gas.
 34. The method as defined in claim 29, wherein said gas is CO andCO₂.
 35. The use of a polymer as defined in claim 1 for vapochromicallysensing the presence of an analyte.
 36. A method of synthesizing apolymer as defined in claim 8 comprising reacting a Cu(II) or a Zn(II)salt with [Au(CN)₂]⁻ in a solvent.
 37. A vapochromic compositioncomprising a metal complex represented by the general formulaM_(W)[M′_(X)(Z)_(Y)]_(N) wherein M and M′ are the same or differentmetals capable of forming a coordinate complex with the Z moiety; Z isselected from the group consisting of halides, pseudohalides, thiolates,alkoxides and amides; W is between 1-6; X and Y between 1-9; and N isbetween 1-5.
 38. The composition as defined in claim 37 wherein; M isselected from the group consisting of Cu and Zn; M′ is selected from thegroup consisting of Au, Ag, Hg and Cu; Z is a pseudohalide selected fromthe selected from the group consisting of CN, SCN, SeCN, TeCN, OCN, CNOand NNN; W and X are 1; Y is 1; and N is 2 or
 3. 39. The method asdefined in claim 23, wherein said analyte is selected from the groupconsisting of ammonia and amines.
 40. The method as defined in claim 39,wherein said analyte is ammonia.
 41. The method as defined in claim 39,wherein said polymer comprises Zn[Au(CN)₂]₂ in any of its polymorphicforms.
 42. The method as defined in claim 39, wherein said polymercomprises Cu[Au(CN)₂]₂.
 43. The method as defined in claim 39, wherein Mis Cu or Zn, M′ is Au, Z is CN, W is 1 and X and Y is
 2. 44. The methodas defined in claim 39, wherein said detecting comprises sensing anychanges in luminescence of said polymer.
 45. The method as defined inclaim 39, wherein said detecting comprises sensing any changes in colourof said polymer.
 46. The method as defined in claim 39, wherein saiddetecting comprises spectroscopically identifying any changes in theinfrared signature of said polymer.
 47. The method as defined in claim40, wherein said exposing comprises exposing said polymer to ammoniapresent in the breath of a patient.
 48. The method ad defined in claim41, wherein said polymer comprises Zn[Au(CN)₂]₂ in its β polymorphicform.